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INVESTIGATIONS ON CESIUM URANATES AND
RELATED COMPOUNDS
0
ACADEMISCH PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR
IN OE WISKUNDE EN NATUURWETENSCHAPPEN AAN
DE UNIVERSITEIT VAN AMSTERDAM OP GEZAG VAN
DE RECTOR MAGNIFICUS DR. G. OEN BOEF, HOOG-
LERAAR IN DE FACULTEIT OER WISKUNDE EN
NATUURWETENSCHAPPEN. IN HET OPENBAAR TE
VERDEDIGEN IN DE AULA DER UNIVERSITEIT
(TIJDELIJK IN DE LUTHERSE KERK, INGANG SINGEL
411, HOEK SPUI) OP WOENSDAG 16 JUNI 1976 OM
15.00 UUR PRECIES
DOOR
ANDRÊ BERNARD van EGMOND
GEBOREN TE HENGELO (O)
Λ t'
Ιί.'Λίά?
Μ
^ ; , * * , , χ
Υ'
—ir-
promotor : prof.dr. B.O. Loopstra
co-promotor: prof.dr.ir. E.H.P. Cordfunke
co-referent: prof.dr. J.A. Goedkoop
pitllfilIpJ
I
If• va**
>
, ",. ---.υ
~'\ '"·*'\ί· aan mijn ouders
voor Ietje
V
-4-
Aan allen, die in enige vorm hebben meegewerkt aan het totstandkomen
van dit proefschrift, betuig ik mijn hartelijke dank.
In het bijzonder dank ik Gernna van Voorst en Anneraieke Berentsen
voor de syntheses van de uranaatpreparaten en Piet van Vlaanderen voor
de hulp bij het Rontgenwerk.
De vakgroep Chemische Fysica van de Technische Hogeschool Twente
zeg ik dank voor het beschikbaarstellen van de PW1100 éénkristaldiffrac-
tometer en de afdeling Fysica van het Reactor Centrum Nederland voor het
gebruik van de PW1150 poederdiffTactometer.
Veel dank ben ik verschuldigd aan mevr.drs. E.M.M. Rutten-Keulemans
die enkele computerprogramma's voor mij toegankelijk maakte»
Veel waardering heb ik voor het typewerk dat verzorgd werd door
mevr. A. Schuyt-.-Fasen en voor de zorg van de reprografische dienst van
het Reactor Centrum Nederland, besteed aan het drukken van dit proef-
schrift.
De direktie van het Reactor Centrum Nederland ben ik erkentelijk
voor de mogelijkheid de tekst van dit proefschrift ook als extern rapport
(RCN-246) te laten verschijnen.
'. -Λ " " .
••;ii ν ; " Α
CHAPTER I
CHAPTER II
""- ' ί- -" '<
• }::.;• ν ζ ^ Μ
Or*.
CHAPTER III
CHAPTER IV
-5-
CONTENTS
INTRODUCTION
1.1. Nuclear technology and cesium uranates
1.2. X-ray diffraction
1.2.1. Single-crystal X-ray diffraction
1.2.2. Powder X-ray diffraction
1.3. Crystallographic computing
References
CHARACTERIZATION OF THE PHASES IN THE Cd-U-0 SYSTEM
2.1. Introduction
2.2. Experimental
2.3. Hexavalent cesium uranates
2.A. The Cs-U-0 system in air
2.4.1. Compositions with Cs/U > 0.5 and < 0.5
2.4.2. The equilibrium C s ^ O ^ * * C s ^ O ^ + ^
2.5. The Cs-U-0 system at low oxygen pressures
References
THE CRYSTAL STRUCTURES OF Cs2U
4O
| 2
3.1. Introduction
3.2. Experimental
3.3. The crystal structure of ct-Cs U 0
3.3.1. Crystal data and intensity measurement
3.3.2. Structure determination of β-0ε2ϋ,0
3.4. The crystal structures of £J- and y-Cs-U.O.-
3.4. ]. Crystal data
3.4.2., The crystal structures of β- and
Y-CS2U
4O
J2
3.5. Discussion
References
THE CRYSTAL STRUCTURES OF C s ^ O ^ AND C s ^ O ^
4.1. Introduction
4.2. Experimental
4.3. Crystal data and crystal structure of Cs.U.O..
4.4. Crystal data and crystal structure of Cs.U,.O,,i J ID
4.5. Discussion
Refe.. Alic
page
9
9
SO
10
12
13
15
17
17
17
18
21
21
22
24
24
26
26
26
26
26
28
29
29
30
31
33
35
35
35
36
38
39
42
im-.'-v.' - • . • ' • f -
Pi·:
H M M
^M
if
CHAPTER V
-6-
THE CRYSTAL STRUCTURES OF
Cs2U15°46 Cs2UO45.1. Introduction
5.2. Experimental
ï,3. The crystal structure of Cs.U 0 ?
5.4. The crystal structure of Cs2U?022
5.5. The crystal structure of Cs U.-O,,
5.6. The crystal structure of CSjUO,
5.7. Discussion
References
CHAPTER VI THE CRYSTAL STRUCTURES OF C s ^ O j
6.1. Introduction
6.2. Experimental
6.3. The X-ray analysis of a-CsXO.
6.4. The X-ray analysis of β-CsJ5J37
6.5. The X-ray analysis of γ-Cs U20_
6.6. The neutron analysis of a-Cs.U„O_
6.7. Discussion
References
CHAPTER VII POTASSIUM AND RUBIDIUM URANATES
7.1. Introduction
7.2. Experimental
7.3. Hexavalent uranates
7.3.1. The potassium uranate system
7.3.2. The rubidium uranate system
7.3.3. The crystal structures of M-U.O-, and
M2U
2O
7 (M - K,Rb)
7.4. Pentavalent uranates
7.5. Discussion
References
CHAPTER VIII CRYSTAL CHEMISTRY OF THE ALKALI URANATES
8.1. Introduction
8.2. Structural characteristics of uranates(VI)
8.3. Uranium(-oxygen) motifs in alkali uranates(VI)
8.3.1. The mono- and diuranate region
8-3.2. The high M/U-ratio region
65
65
65
65
65
67
67
69
69
71
73
73
73
78
80
81
' " '" ül \irn
-7-
8.3.3. The 3D-structure region
8.3.4. Cs2UI5O46 and the M ^ O ^
(M - K,Rb,Cs)
8.3.5. The tetraurar.ate region
8.4. Uranium-uranium distances and uranium-oxygen
bonding
8.5. The composition of the uranium layers
8.6. Uranium layer in Cs2U,O._ and Cs-U5O.6
8.7. Influence of the metal radius on the interplanar
distances in layer structures of uranates(VI)
8.8. Uranates(V) with the perovskite structure
References
APPENDICES
SAMENVATTING
CURRICULUM VITAE
page
8!
82
83
84
86
90
91
94
95
98
122
124
it'!
\A:t • , ''.-
3&J:
, . • · > . . < · , ,
'7 '
Η . •'-''
which the
239
238,
-9-
CHAPTER I. INTRODUCTION
1.1. Nuclear technology and cesium uranates
Uranium has been successfully applied in nuclear technology as a source
of energy since the Second World W;ir. In most of the nuclear reactors
235the isotope U - which occurs for only 0.7% in natural uranium - is
used. Therefore nuclear technologists have looked for a method to harness
the greater part of the available uranium, the U isotope, for the
production of energy. This can be achieved in a fast-breeder reactor, in
U isotope is converted into the fissile plutonium isotope
Pu. The construction of a prototype reactor is a joint project of the
governments of Western-Germany, Luxemburg, Belgium and The Netherlands.
The work described in this thesis has been partly performed within the
scope of this fast-breeder reactor project.
During fission of the nuclear fuel many elements are formed such as
iodine, molybdenum, rubidium, cesium [I], Some of these elements will
diffuse from the hot centre of the (U,Pu)-oxide pellets (2500°-2900°C)
to the cooler parts of the fuel pins (50O°-700°C) [2,3,4]. In the re-
sulting medley of elements three types of chemical reactions can occur:
1. reactions between fission products mutually, mainly producing ir.olyb-
dates [5,63;
2. corrosion of the stainless steel pin cladding by fission products,
forming chromates [7,8];
3. reactions between fission products and the nuclear fuel (U,Pu)-
oxide, resulting in the formation of uranates [8,9] and piutonates.
Since cesium is found in substantial quantities among the fission
products [1] cesium uranates will also be formed during irradiation I 10}.
The formation of cesium uranates appears to be strongly dependent on the
(partial) oxygen potential in the fuel rod [2,3,10], and may rlfly a s
ifc~
nificant role in the swelling of the fuel and even cause fuel pin failure
19], Therefore a study of the Cs-U-0 system is clearly called for.
In addition, uranates are of great interest to structural chemistry,
As early as 1935 Frankuchen demonstrated I In·· existence of the linear con-
2+figuration (O-U-0) in the crystal structm >I solium uranyl acetate
[J 11. Since that time many uranium compound.·- e 'ujen shown to contain
this uranyl group. In I960 Kovba investigatec o regularities in
uranate structures [121, while Keller consider*d some uranates in an ex-
-10-
ι -C - , --'
tensive structural study on actinide compounds some years later [13].
However, both authors had no detailed structural information on cesium
uranates available. In addition many investigations concerning the other
alkali uranates, frequently contradicting earlier statements and even
mutually conflicting, have been published since the last decade.
In this thesis the crystal structures of the cesium uranates are
described, whereas the rubidium and potassium uranate systems are re-
investigated. Finally the structural information on the alkali uranates
is classed in a way similar as Kovba and Keller did for earth alkaline
uranates mainly.
1.2. X-ray diffraction
One of the commonly adopted techniques to obtain structural information
from solid compounds is X-ray diffraction. Since numerous authors have
described both theory and practice of X-ray diffraction, only some sum-
marizing remarks will be made here, chiefly for a clear apprehension of
the description of the computer programs used in this study.
/..·;.ƒ,·';••
•mm
ii^^^Single-crjrs tal_X-ray_dif fraction
When a rotating single crystal is exposed to an X-ray beam a pattern of
diffracted beams is obtained. The direction of the weak diffracted beams
is fixed by the orientation of the crystal, the size of the unit cell
from which the crystal can be thought to be built, and the wavelength of
the applied X-rays. The angle 2Θ between the diffracted beam and the
beam passing through the crystal is given by
2 d sin θ = λ (1.2.1)
where λ is the wavelength of the X-rays, and d is the interpianar spacing
of the reflection planes. The value d can be calculated from the unit
cell constants by
2A' + k
2B' + 1
2C' + 2klD' 21hE' 2hkF' (1.2.2)
in which A', B', C', D', E' and F'.are related to the unit cell para-
meters a, b, c, a, 0 and γ, and h, k, 1, the so-called Laue-indices, can
f"?/• "--• •
.-ir" •; -
-•< •
m
Η Λ-11-
forra any combination of three integers. The intensity of the diffracted
beam I is fixed by the atomic arrangement in the unit cell. The
atomic arrangement, i.e. the electron density ρ at a point r = (x,y,z)
in the unit cell with volume V can be described by a Fourier summation
p(r) Γ Σ F- exp(-2iTih.r). (1.2.3)
The summation has to be carried out for all \, issible reflections with
indices h = (h,k,l), whereas F^ is the so-called structure factor asso-
ciated with a reflection h.
The absolute value of the complex structure factor, F , • |Fe-|, is
measurable by X-ray diffraction, for the intensity of a diffracted beam,
I , , can be written asobs
j: • : - ,
I , ~ L ρ A F , .obs
r obs
(1.2.4)
In expression 1.2.4 L, ρ and A denote the Lorentz, polarization and ab-
sorption correction factors, which in general depend on the diffraction
angle 2Θ and the applied X-ray diffraction technique.
.The phase of the complex number F^ cannot be determined directly from
the X-ray intensities. The determination of the phase angles constitutes
the main problem in X-ray crystallography.
From a given electron-density distribution the structure factor can
also be calculated by
= P(r) exp(2irih.r). (1.2.5)
As the electron density in the unit cell can be described by a set of
atomic coordinates, the volume integral 1.2.5 can be replaced by a sum-
mation series over discrete atomic positions, yielding a calculated
structure factor F^. The calculated phase of this structure factor can
be assigned to the F , -value for use in equation 1.2.3. An absolute
value F is also associated with the calculated structure factorF . » |
from equation 1.2.5.
In general a set of atomic coordinates is considered to give the
correct description of a crystal structure when the calculated structure
factors {F . } and scaled observed structure factors {F , } agree one by
one. However, discrepancies between the two sets of structure factors
Λ i>:
m
: • • · • • * •
-12-
always will exist. Therefore a process is needed which determines atomic
positions for which the F . -set fits the F . -set best. For this pur-
pose use is often made of a so-called least-squares minimization: the
atomic position parameters are varied until the expression
Ζ W|F
h
- Fobs calc
1 (1.2.6)
reaches its minimum value. The symbol w stands for an appropriate
weighting factor in the least-squares process. Since the structure fac-
tor expression 1.2.5 is not a linear function of the atomic coordinates
a starting set of the coordinates is needed, which is then refined by the
above mentioned least-squares procedure.
Finally from the obtained atomic positions conclusions can be drawn
about interatomic distances and angles, the coordination of atoms, etc.
dif fraction
In the foregoing it was assumed that .single crystals of the compound to
be studied with dimensions suitable for single crystal X-ray diffraction
(0 = O.I mm) can be obtained. Unfortunately many compounds are only
available in powdered form. Then powder diffraction data may be used for
a structural study, although some complications are met in comparison
with single-crystal diffraction.
The random orientation of the particles in a powder sample causes
th3 diffracted radiation to be distributed over a cone from which the
diffraction angles 2Θ can be obtained, either photographically (Guinier,
Debeye-Sherrer) or by step-scan counter methods. With the aid of the
formula 1.2.1 the 20 angles can be turned into d-values. The charac-
teristic d-values of a sample are converted to the so-called Q-values
according to
10
From 1.2.2 it follows that
(1.2.7)
2 2h A + k Β 1
2C + 2klD + 21hE + 2hkF (1.2.8)
in which the symbols Α,Β ... F are the tenthousandfold of the cor-
f.
9*'
-13-
•!./·?
responding primed quantities in 1.2.2. The obtained Q-values form a set
of numbers which are well-suited for the indexing of the pattern, i.e.
to trace the quantities A ... F that constitute the basis of the Q-set.
Once this regularity is known the unit cell can be calculated.
Another complication in powder X-ray diffraction concerns the overlap
of the diffracted beams. Not only symmetry-equivalent reflections diffract
in the same cone but also reflections with little difference in 2Θ overlap
in the X-ray pattern. The first fact causes the introduction of a multi-
plicity factor η in the intensity formula 1.2.4, whereas the second fact
raises difficulties in assigning the proper intensity to one particular
reflection. Often it is only possible to assign an intensity I , to a
group of j reflections. Therefore expression 1.2.4 has to be changed to
I , = Σ η. L. p. A. F , .obs . j j
Hj j obs.j
(1.2.9)
·:/?*
which yields, after reduction with an average correction factor, the
2quantities 2. n. F , ..In addition the least-squares criterion 1.2.6
J J obs.jhas to be changed to
Ζ w \l n. F2, . - I n. F
2 , .I
2 (1.2.10)
'j J obs.j j j calcj1
where the first summation is taken over the reflection groups.
1.3. Crystallographic computing
During the work described in this thesis the following programs have
been used on the CDC-660Ö computer of Reactor Centrum Nederland.
F15 calculates corrections for non-proper alignment of equipment, film-
shrinkage and zeropoint from standard lines (commonly α-quartz) in
powder patterns and applies these corrections to sample lines to
yield Q-values. This program has been written by Rietveld [14].
VISSER tries to find the unit cell from the corrected Q-values of a com-
pound. The method was given by Runge Γ15], rediscovered by Ito [16],
refined by De Wolff [17] and programmed by Visser [18]. In short
the program tries to recognize regularities in the Q , -set and to
obs
combine two-dimensional zones of reflections to three-dimensional
lattices.
I-·?. .*'•
-14-
TAUPIN, a similar program for indexing of powder patterns, has been
coded by Taupin [193. In a trial-and-error method a system of six
equations of type 1.2.8 is solved for the lower Q-values and cer-
tain low h, k and 1-values. Other Q's are tried to fit the computed
cell. The program was written in IBM-Fortran, which gives some
problems upon conversion to other systems. The computing time is
high compared to the Visser program.
TI23 refines the unit cell by a least-squares procedure on 40 Q-values
at most, based on the linear expression 1.2.8. The program has been
encoded by Rietveld [14].
PWII00. Intensities of a single crystal were measured on the PW1100
computer-controlled single-crystal X-ray diffractometer of the
Chemical Physical Laboratory of Twente University of Technology.
A program, also called PW1I00, was used to correct the intensities
according to expression 1.2.4 [203.
SFLS, a structure-factor least-squares program, performs all refinements
of structural parameters. The program has been coded by Mrs. Rutten-
Keulemans [21]. The program is suited for single-crystal computa-
tions (expressions !.2.5 and 1.2.6) as well as powder computations,
(expression 1.2.10). In the latter case SFLS has been adapted to
the input requirements of the FOUR-program: the quantities
2
Σ. n. F , . (expression 1.2.9) are split into F , -values propor-
tional to the calculated structure factors F , .
calc
FOUR performs Fourier summations in X-ray crystallography, of the type
as given in (1.2.3). The program was written by De Graaff and
Mrs. Rutten-Keulemans [21].
T418 performs similar calculations as SFLS but for neutron-diffraction
data. It has been coded by Rietveld [22], The least-squares refine-
ment is applied to the separate step-scan intensity values, per-
forming a so-called profile least-squares analysis. This procedure
is only possible when the intensity peak shape is accurately known:
neutron diffraction peaks have a Gaussian shape.
X-RAY SYSTEM. This system of programs contains a compatible set of many
crystallographic computing features [23], but is not suited for
computations on overlapping powder intensities. The system has been
used for the single-crystal studies and for computation of inter-
atomic distances and angles.
Stv;
, * —/ f,*fl
• . •' \'
• • ; . · • « · • · . -
-'. ν Κ'.
-15-
STEREO calculates stereo-plots of crystal structures. The program is an
enlarged local version of a program written by Van de Waal [24],
References
[1] F.L. Lisman, R.M. Abernathey, W.J. Maeck and J.E. Rein,
Nucl. Sci. Eng., 42, 191 (1970).
[2] E.A, Aitken, S.K, Evans and B.F. Rubin, in "Behaviour and Chemical
State of Irradiated Ceramic Fuels", proceedings of a panel (1972),
I.A.E.A. Vienna, p.269 (1974).
L3] M.G. Adamson and E.A. Aitken, Trans. Amer. Nucl. Soc, l]\ 195 (1973).
[4] L.A. Neimark, J.D.B. Lambert, W.F. Murphy and C.W. Frenco,
Nucl. Technol., J£, 75 (1972).
[5] C.E. Johnson, N.R. Stalica, C.A. Seils and K.E. Anderson, U.S. Atom.
Energy Conm. Rep. No. ANL-7675, p.102 (1969).
[61 C.E. Johnson, I. Johnson, C.A. Seils, K.E. Anderson, G. Staahl and
C. Wach, U.S Atom. Energy Conm. Rep. No. ANL-7877, p.29 (1972).
[7] J.W. Weber and E.D. Jensen, Trans. Amer. Nucl. Soc, JU_, 175 (1975).
[8] P.S. Maiya, U.S. Atom. Energy Coma. Rep. No. ANL-7833, p.5,11 (1971).
[9] L.A. Neimark and J.D.B. Lambert, U.S. Atom. Energy Conm. Rep.
No. ANL-RDP-11, p.6,18 (1972).
[10] I. Johnson and C.E. Johnson, Trans. Amer. Nucl. Sci., JJ7, 194 (1973).
[II] I. Frankuchen, Z. Kristallogr. 9^, 473 (1935).
[12] L.M. Kovba, Izvest. Vysshikh Ucheb. ZavedeniT,
Khira. i Khim. Tekhnol. _3» 219 (I960); see Chem. Abstr. 54, No. 21900
(I960). ,
[13] C. Keller, Habilitationsschrift, Technische Hochschule Fredericiana,
Karlsruhe (1964); also KFK-225, Karlsruhe (1964).
[14] H.M. Rietveld, crystallographic computer programs RCN, private com-
munication (1972).
[15] C. Runge, Z. Physik, Jji, 509 (1917).
[16] T. Ito, Nature, J6j4, 755 (1949).
[17] P.M. de Wolff, Acta Cryst., 10, 590 (1957).
• j ,
1 Λ ·!
ϊ-¥---
JL
«*·>'.
...*-16-
[18] J.W. Visser, J. Appl. Cryst., 2, 89 (1969).
Π 9 ] D. Taupin, J. Appl. Cryst., b, 380 (1973).
[20] A.B. van Egmond, Direkte methoden in de kristallografie, Twente
University of Technology (1973).
[21] E.W.M. Rutten-Keulemans and R.A.G. de Graaff, Handleidingen bij SFLS
en FOUR, University of Leiden.
[22] H.M. Rietveld, J. Appl. Cryst., 2, 65 (1969).
[23] The X-ray system - version June 1972. Technical Report TR 192 of
the computer science centre, University of Maryland, Dutch version-
update September 1973.
[24] B.W. van de Waal, progress report Chemical Physical Department
No. 2, Twente University of Technology, p.H98-106.
y
Μ".
& ' •
-17-
CHAPTER II. CHARACTERIZATION OF THE PHASES IN THE Cs-U-0 SYSTEM **
2.1. Introduction
Extensive studies have been made on the alkali-metal uranates, especially
by Ippolitova et al. [2], indicating the existence of a number of poly-
uranates, depending on the temperature and the M/U-ratio, where Μ is one
of the alkali metals lithium, sodium, potassium, rubidium and cesium.
But although since that time (c.1960) improved methods of investigations
have led to some refinements [3,4,5] and also other investigators have
studied alkali uranate systems [6,7,8], the literature data concerning their
composition and crystal structures is still incomplete or even conflicting.
Interest in these compounds as possible reaction products in nuclear
fuel elements during fission has grown in the last decade and has given
the impetus to reinvestigate these alkali uranate systems systematically.
In particular cesium plays a role in the in-pile performance of oxide
fuel pins. It is therefore important to have a detailed knowledge of the
possibility of cesium-uranate formation in dependence of the partial
oxygen pressure of the fuel. In this chapter a rather detailed investi-
gation of the Cs-U-0 system is presented.
2.2. Experimental
Crystalline samples of the various cesium-uranate phases were prepared
by heating carefully ground mixtures of amorphous UO, and cesium car-
bonate in a gold boat in air at 600 C, until reaction was complete.
The U03 had been obtained by decomposition of hydrated UO,, whereas in
some experiments also UO., which was formed upon heating uranyl nitrate,
was used. In a few cases cesium nitrate was used as a reagent instead of
cesium carbonate.
The atomic ratios Cs/U were accurately fixed in the starting
materials and ranged from 4.0 down to 0.05. In table 2.1 the Cs/U-ratios
of all prepared mixtures are collected. Since reaction proceeded slowly,
especially at low Cs/U-ratios, the reaction mixtures were reground
*) This chapter is a revised and extended version of the paper by
E.H.P. Cordfunke, A.B. van Egmond and G. van Voorst, J. inorg. nucl.
Chem., 37, 1433 (1975).
'„• £ Λ, ' . »"ΐλ ir'··
ïrJK&^£.·*.'
-18-
Table ?..!.
Cs/U-ratios of the. samples prepared during thin investigation.
0.05
0.075
0.10
0.125
0.15
0.20
0.22
0.235
0.25
0.265
0.27
0.286
0.30
0.33
0.36
0.375
0.40
0.42
0.435
0.45
0.475
0.50
0.60
0.667
0.75
0.775
0.78
0.80
0.83
0.857
1.0
1.5
2.0
2.5
3.0
4 . 0
several times between the heating periods. Progress of the reaction was
checked by taking X-ray diffraction exposures on a Nonius Guinier focus-
ing camera, using monochromated CuKa radiation. In general equilibrium
was obtained after a heating period of one week.
Thermal stabilities were investigated by static experiments
- ignition of the sample at a fixed temperature during various periods
of time - and by differential thermal, analysis (DTA) using a BDL-apparatus
with a heating rate 9 /roin* Exposures taken on a Nonius high-temperature
X-ray Guinier-Ill camera and thermogravimetric analysis (TGA), using a
Stanton thermo-balance, completed the DTA-results.
The composition of the phases formed was analyzed for U(VI), U(V)
and cesium content. The uranium content was analyzed potentioinetrically
after dissolution of the sample in concentrated phosphoric acid, according
to the procedure described by Eberle et al. [9]. The cesium content was
determined by titration of a buffered solution (pH = 5) with sodium
tetraphenyl borate.
Densities of powder specimens were measured pycnometrically in
diethyl phtalate, n-dodecane or dioctyl phtalate at 20 C. The density
measurements were carried out at least in threefold.
2.3. Hexavalent cesium uranates
From the Guinier X-ray patterns distinct cesium uranates have been found
to exist in air upto 600°C at atomic ratios Cs/U - 0.133, 0.286, 0.40,
0.50, 0.80, 1.0 and 2.0. Since uranium is entirely in the hexavalent
state, as followed from chemical analysis, the following formulae must
be assigned to these atomic ratios: C S
2U| 5 ° A 6 '
ε 82
υ7°22'
Cs2
US°16'
Cs2U
4O
J 3, Cs
4U
5O
] 7, Cs
2U
2O, and C s ^ .
**t'^ï**t:
:t -
-19-
As described in section 2.2 the formation of the cesium-uranate
phases is very slow, especially at low Cs/U-ratios. Notwithstanding
long heating periods (several weeks) and repeated grinding of the reac-
tion mixtures, it appeared to be impossible to obtain the brown-orange
coloured Cs„U_O„„ phase in a pure form: it was always contaminated with
both neighbouring phases. Previously Efremova et al. [10] have found the
CsΛ5-0 phase but it was assigned the formula Cs-lLO.g. Since a dif-
ference in chemical composition of the reaction mixtures with
Cs/U = 0.286 and Cs/U * 0.33 can readily be detected on X-ray exposures,
the Cs„U,O1£. formula cannot be correct. More important is that the
Cs υ?0 . phase appeared to be isostructural with
K
2
U7°22
o f w n*
c n a
detailed structural study has been published by Kovba [II]. This also
supports the assignment of the formula Cs.l^O.o·
The brown-grey cesium-uranate phase with the lowest Cs/U-ratio has
not yet been described in the literature. It should be noted that its
X-ray pattern always contained lines of V-0o and Cs„ü_O__ even after
long heating periods. At first only a rough composition ratio between
0.10 and 0.15 could be assigned, and merely from observations on the
crystallinety of the sample mixtures the formula Cs„U.,0,„ (Cs/U=0.125)
was decided [1], However, as will be described in chapter V, the formula
Cs„U..O,, (Cs/U=0.133) is probably the correct one for this phase.
The compounds CsJ 0 , and Cs„U,O._, both yellow-brown coloured,
behave very similarly. After formation of these phases at 600°C their
X-ray patterns are rather diffuse, which indicates a poor crystallinity.
On heating these phases crystallization occurs above 700 C. The pattern
of Cs-U_O,, is contaminated with lines of Cs„U,O„„ and Cs-U.O._. The
density of Cs„U,0 _ was measured to be 6.8(2).
Cs.U 0 is a bright-yellow phase, which has not.been described
previously. Instead a phase with formula Cs Ü 0.» has been suggested by
Efremova et al. [10]. However, the latter phase could not be detected in
this investigation. In experiments to synthesize single crystals of
cesium uranates single crystals of CsASJi - were often obtained, as will
be described in chapter V. The measured density of Cs,U_0._ is 6.6(2).
Cs„U.O7 has a pink-yellow colour. When heated above 300°C it trans-
forms slowly into a slightly different phase that can easily be frozen in
at room temperature upon rapid cooling. The compound, stable below the
transition temperature, is called a-Cs2U„0_, while the salmon phase,
stable at higher temperatures is referred to as 6-Cs?U_O7.
ν Ι Λ • ft.· '- '^ y'·"'^' '•
Γ . . . . . . . ,
— ipl#?;
-20-
1200
1100
1000
900S
Sa
υ,ο.
υ,ο,
a" 3/
C*,U40„
700
600
500
400
,300
200
100
u,o.
ί1
υ
c»«u io1 7
C«,K,O„
ΙΛ,Ο,Ο,
uCt,U2O7
+
Cs,UO4
0.5 1.0
Figure 2.1.
15 2.0C»/U. atomic ratio
The tentative phase diagram of the pseudo-binary Ce-U-0
system in air. The dotted linea represent metastable equi-
libria in open atmosphere with p(CsJD) = 0. The diuranate
•γ-CsJJJ)- is metastable and has therefore not been indicated
in the diagram.
.', 11 f. . τ
••-•te i fc
-21-
A third form of Cs„U.O7 could also be synthesized at 600 C, when
the synthesis is carried out with U0. prepared by decomposition of
uranyl nitrate in vacuum. This phase, γ-Cs U„0_, however, was always
contaminated with the β-diuranate. In general the reaction starting
with U0„ from uranyl nitrate is faster than the reaction starting with
Ü0. produced by decomposition of hydrated UO^. At 800°C y-Cs^u^
rapidly transforms into g-Cs-lLO., indicating that y-CSpU,^ *-s t n e
metastable compound. In succeeding experiments the reaction time, the
temperature and the concentration of nitrate impurities were varied,
but the γ-diuranate could not be synthesized in a pure form. Although
y-Cs2U
2O
7, previously described by Kovba et al. [12], and B-Cs
2U
2O
7>
recently studied by O'Hare and Hoekstra [13], were reported to be
hygroscopic, a reaction with water vapour could not be established at
room temperature in case of any of the three uranate phases. The den-
sity of B-Cs.U-O. has been measured yielding a value 6.5 (I).
The orange-coloured Cs_UO, is very hygroscopic in agreement with
the observations of Efremova et al. [10] and Kovba et al. [12]. There-
fore it has been handled in a dry box.
A uranate with a Cs/U-ratio 4.0 has been reported by Efremova et
al. [14]. In this investigation no indication for the existence of the
"mesouranate" Cs.UO,. has been found, in agreement with the results of
Kepert [15] and Hoekstra and Siegel [16].
2.4. The Cs-U-0 sysizem in air
In figure 2.1 the phase relations in the Cs-U-0 system are represented.
In this figure some lines have been dotted because they represent meta-
stable equilibria at p(Cs.O) * 0. In the next sections a description
of this phase diagram is given.
f-.'V:
Cesium uranates with Cs/U-ratio > 0.5 decompose into Cs„0 and the neigh-
bouring uranate with lower Cs/U-ratio at a rate dependent on the tempe-
rature and the Cs/U-ratio. When heated in air Cs-UO, already decomposes
at 650 C into ft-C&JUJ^j» which *-n turn decomposes into Cs,U.O-
7 at
900°C. At 1000°C Cs.U-O.y decomposes very slowly into CSjU-O.».
sir
' V 'Λ1 a· \
-22-
i r " When cesium uranates with Cs/U-ratio <0.5 are heated in air a new
compound is formed above 730 C. In this compound which has the Cs/U-
ratio = 0.22, as follows from the X-ray analysis, uranium is not entirely
in the hexavalent state. The U(IV)/U(VI)-ratio is 0.126 as determined
by chemical analysis. Together with the cesium content this leads to
the formula Cs U 0 . When Cs.U 0 is heated above 875°C a new compound
with Cs/U • 0.33 can be formed. Together with the U(IV)/U(VI)-ratio
(0.20) the formula Cs.U O . can be deduced for this uranate. It is notI 6 18
stable in open atmosphere: upon heating it decomposes into U-Og under
evaporation of Cs.O.
Cs-U^O., starts to crystallize when heated above 700 C. The phase,
which is formed then, appears to have a homogeneity range. The phase
width, as determined from the X-ray exposures of compounds with different
Cs/U-ratios, which were heated at various temperatures between 700
and 1000°C, ranges from 0.400-0.435 (750°C), 0.375-0.450 (800°C) to
0.375-0.475 at 95O°C. Above !000°C the "Cs^O^-phase" reaches the
limiting composition Cs/U = 0.5, i.e. Cs„U,O.~.
2.4.2. The equilibrium C s ^ O ^ ±5. C s Λ Ο ^ + j02
When Cs?U,0 _ is heated a reversible dissociation according to
CS2
U4°,3
CS2Ü4°12
has been found. The oxygen pressure of this equilibrium is strongly de-
pendent on the temperature, as observed by Cordfunke [17]. In air
Cs D O . is stable up to 1040°C, CsJ 0 then formed up to at least
1250 C. This phase in turn then decomposes into UO and gaseous mix--13
tures of cesium and oxygen [18], At low oxygen pressures (p(0„)< 10 atm)
dissociation of CSjU 0.„ already occurs at 600°C.
The formation of Cs-U.O., as a result of the calcination of "cesium
mesouranate Cs.UO." (see section 2.3) has been observed by Efremova
et al. [143. They also found a reversible transition into Cs_U,O._ upon
cooling Cs.U.O down to 500°C in air. However the X-ray pattern, re-
ported by these authors, does not agree very well with the data resulting
from this investigation.
ίΤΛ.
i
X
i .·.- ι.
-23-
1200
1100
1000
I 900
a
I 800
7 0 0
600
500 °»
400
300
200
too
-
-
i
'i
ê
C,U4O,2
o"a"
Μδ
•
ο*3
υ
ο
*
Ιυ
êίυ
+
υ
C^lbOe
C ^ U 4 O l 3
sc^o,4-
C^,U04
4
CS2UO4
i
0.5 1.0 1.5
C*/U . atomic ratio
2.0
Figure 2.2.
The tentative phase diagram of the pseudo-hinary Cs-U-0
system at low oxygen pressure (p(0 ) < / ' aim).
The phase transitions of CsJ}.0no at β'. ' and 695 C have
not been indicated in the diagram (take:· fvom L171).
i t -•, "5.
if' -
fo-
Ί , t
' - . ; • • $
\
-24-
2.5. The Cs-U-0 system at low oxygen pressure
As stated above Cs.U O dissociates into Cs.U.O „ and 0„ at 600°C when
the oxygen pressure is sufficiently low. Moreover, all cesium uranates(VI)
decompose slowly into UO-, Cs2U,O.
2 and Cs
?O(g) above 600 C. Therefore
the upper part of the phase diagram of the Cs-U-O system at low oxygen
pressure is rather simple, as can be seen in figure 2.2.
The compound Cs2U,O
)2 exhibits two phase changes before decomposi-
tion into U0„ and Cs_O(g) [18]. The different phases will be called a-,
β- and y-CsJü.O y. The α -*• 3 transition occurs at 625°C and the β •*• γ
transition at 695°C as has been found from high-temperature X-ray work
(see chapter III) and DTA experiments [17]. The density of the α-phase
has been measured to be 7.2 (1).
As Cs2U,O._ is the only stable cesium uranate at low oxygen pressures
and elevated temperatures, there is clear evidence that it is the cesium
uranate which is formed in reactor fuel pins during fission (see section
1.1).
References
[1] E.H.P. Cordfunke, A.B. van Egmond and G. van Voorst,
J, inorg. nucl. Chem., y}_, 1433 (1975).
[2] Investigations in the field of Uranium Chemistry, a symposium of
papers, edited by V.I. Spitsyn, Pub. House Mosc. Univ., (1961),
translated and released by Argonne National Laboratory, Illinois,
Rep.no. ANL.-Trans.-33, (1964).
[3] L.M. Kovba and V.I. Trunova, Radiokhimiya, _Π· 7 7 3
(1971).
[4] L.M. Kovba, Radiokhimiya, JJ2, 5 2 2
('970).
[5] L.M. Kovba, Russ. J. Inorg. Chem., J^, 1639 (1971).
[6] J. Hauck, J. inorg. nucl. Chem., 3£, 2291 (1974).
[73 J.S. Anderson, Chimia, 23, 438 (1969).
[8] E.H.P. Cordfunke and B.O. Loopstra, J. inorg. nucl. Chem., 33_,
2427 (1971).
[9] A.R. Eberle, M.W. Lerner, C.G. Goldbeck and C.J. Rodden, NBL-report,
no. 252 (1970).
t! •)>.
•<· 'φ
f;-a
ι,;, Λ.-,-
ΑίΤ-Ι
ff'T
/•Μ::
-25-
[10] K.M. Efremova, E.Α. Ippolitova, Yu.P. Simanov and V.I. Spitsyn,
Dokl. Akad. Nauk. SSSR., 2A„ 1057 (1959).
[11] L.M. Kovba, Zh. Strukt. Khim., _Π, 256 (1972).
[12] L.M. Kovba, I.A. Murav'eva and A.S. Orlova, Radiokhimiya J_6, 648
(1974).
[13] P.A.G. O'Hare and H.R. Hoekstra, J. Chem. Thermodynamics, T_, 831
(1975).
[14] K.M. Efremova, E.A. Ippolitova and Yu.P. Simanov, Vestn. Mosk.
Univ., Khiro. Ser.t 24, 57 (1969).
[15] D.G. Kepert, Thesis, University of Melbourne (1960).
[16] H.R. Hoekstra and S. Siegel, J. inorg. nucl. Chem., 2£, 693 (1964).
[17] E.H.P. Cordfunke, in "Thermodynamics of Nuclear Materials",
proceedings of a symposium (1974), I.A.E.Α., Vienna, Vol. II, p.185
(1975).
[18] E.H.P. Cordfunke and G. Prins, Fast Reactor Programme Fourth
Quarter 1974 Progress Report, RCN-2°5, p.39 (1975).
i .
Η • C
Ï'.·:•••-•-."••·
[i
-26-
CHAPTER III. THE CRYSTAL STRUCTURES OF Cs2U
4O
] 2 '
3,1. Introduction
In chapter II of this thesis it was concluded that Cs U^O 2 is formed in
reactor fuel pins during irradiation. Therefore this study of the struc-
tures of cesium uranates starts with the description of the structure of
the technically most-important compound Cs2U,0 „.
In the foregoing chapter it also appeared that Cs-U.O „ has three
different phases, which were called α-, β~ and γ-Cs U 0 . The transitions,
α + 6 at 625°C and 0 ·+ γ at 695°C, are reversible. The γ-phase slowly de-
composes into U0_+ at a temperature depending on the oxygen pressure.
In figure 3.1 a high-temperature X-ray film of a Cs ϋ,Ο., sample, heated
in nitrogen, has been reproduced, showing the phase transitions and the
final decomposition of Cs_U,O._.
ν" '5·'^
Ιΐ
3.2. Experimental
Single crystals of Cs„U,Oj0 were grown at about 1150 C from molten cesium
sulphate on amorphous UO- in a platinum cup. Black pyramidal crystals
were formed after 45 hours. The crystals, which were partly clustered,
were washed from bulk cesium sulphate with ethanol and methanol. Attempts
to grind the jagged crystals in a crystal spherizer yielded ellipsoidal
crystals.
Powder samples of Cs.U.O „ were prepared by calcination of Cs-U,0 .
samples in inert atmosphere, as described in chapter II,
* :~jjy
3.3. The crystal structure of a-Cs.U.O „
3. 3Λ1 i_Cr2£ ta l_da ta_and_in t ensi t j;_mea sur emen t
Single crystals of a-Cs-U.O,», mounted on a Philips PW1100 computer-
controlled single-crystal X-ray diffractometer, showed a pseudo-cubic
unit cell. Due to the rapid cooling down to room temperature to avoid
oxidation of the crystals to Cs_U,0._ the crystals were of a poor quality.
*) This chapter is a revised and slightly extended version of the paperby A.B. van Egmond, J. inorg. nucl. Chem., 3Τ_, 1929 (1975).
''<· · .: * c
t r-'-.
'' ί V* ,'-'ν'• -,- 'x\,.y.:,./.,·,_ ,••'-•'"
5ΟΟ-
700-
900-
°c
"SS5ίf^.^
'2Θ25
5?
α:
L Ü
Figure 3.1.
High-temperature X-ray film of CsJJ 0. , showing the X-ray patterns of the
three CsJi.O^ phases and U0„+: a •* β transition at 626 C,
β -> γ transition at 695°C, y •* U00 decomposition at - 900 C.
(Nonius Guinier-III camera, monoehromated CuKa. radiation)
:m
-28-
The unit-cell parameters were determined from a powder specimen, mounted
en a PWII50 powder diffTactometer using CuKa radiation and quartz as an
internal standard. A least-squares refinement on Q-values led to a
rhombohedral cell with a - 10.9623(6)8 and α « 89.402(7)°. The calculated
density 7.110, based on four Cs„U,012 units in the cell, is in good
agreement with the measured density 7.2(2), determined pyenometrieally
in diocthyl phtalate.
Thereupon 2635 reflections were measured from a single crystal
using monobromated ΜοΚα-radiation in the range 2°<θ<20°. The crystal
was an irregular sphere with a diameter of 0.16 mm. As there were no sys-
tematic extinctions in the reflection set the space group of a-Cs.U.O.-
can be R3, R3, R3m, R32 and R3m [1]. Because a significant difference
in intensity between reflections hkl and khl could not be detected, the
space group is limited to R3m, R32 or R3m. The reflection set was
averaged, yielding 478 independent reflections, 398 of which were sig-
nificantly greater than three times the average counting standard
deviation.
3.3.2. Structure determination of ct-Cs-U.O „
The space group of a vector set (Patterson function) corresponding to
R3m, R32 and R3m, itself is R3m [2]. The rough positions of the uranium
atoms were derived from a three-dimensional Patterson synthesis. After
refinement of these positions in space group R3m and calculation of a
Fourier map cesium atoms could be located. Isotropic refinement in R3m
resulted in an R -factor of 21.55!. Refinement of the similar structure
in space group R32 gave an R -factor of 17.9% and finally refinement in
space group R3m resulted in an R -factor of 14.92. In the refinements
scattering factors were taken from Cromer and Waber [3], corrected with
&£' for anomalous dispersion, taken from Cromer [4],
From Hamilton's test of the R-factor [5,6] the space group of
a-Cs2U,0.- was concluded to be R3m, with a reliability better than 99.5Z.
By including a spherical absorption correction, anisotropic refinement
of the metal atoms and omitting three heavily weighted reflections
the R -factor dropped to 10.8Z. The final coordinates of o-Cs_U,0._ are
*) • ο 2 4R » [ £ w A F / Ε w F , ] 2 , weights from s ta t i s t ic s ,
w obs
t •- '-' Λ
i
# ·
-29-
listed in table 3.1. As three of the nine anisotropic temperature
factors were non-positive definite, these factors have not been tabu-
lated. A list of observed and calculated structure factors is given in
appendix A3; the powder X-ray pattern of a-Cs V 0 is listed in appen-
dix C4.
Table 2.2.
Least-squares coordinates of u-Cs;>Ui
.0 (RSm).f 1 Ci
ATOM
UI
U2
U3
U4
Csl
Cs2
Cs3
Cs4
x/a o(x/a)
0.000
0.500
0.242
0.733
0.235
0.369
0.660
0.122
0.869
y/b o(y/b)
0.003
0.002
0.002
0.004
0.004
0.007
0.004
0.003
3.4. The crystal structures of β- and Y-CS„U,0 „
The splitting of the powder reflections of B-Cs.U.O.- (see figure 3.1)
could be explained by a slight distortion of the rhombohedral cell.
Thia distorted cell turned out to have two equal axes and angles,
a = b = 11.235(2)8, c « 10.793(2)8, a = 0 = 88.17(2)°, γ - 90.83(2)°,
and to be C-centered. Therefore the cell symmetry is monoclinic; taking
half of the diagonals of the triclinic ab-plane, the monoclinic unit
cell can be deduced to have the following parameters: a « 7.886(1)8,
b « 8.002(1)8, c - 10.793(2)8, β • 92.62(1)°. Systematic extinctions,
(0k0 absent for k * 2n+l) correspond to the space groups P2./m and P2..
The X-ray pattern of Y-CS-U.O.- is much simpler than that of «-
and B-Cs-U.O.» as can be seen from figure 3.1. The pattern of Y-CS.U,0.2
can be indexed on a cubic cell with a - 11.2295(6)8. The space group of
this cubic unit cell is Fd3m, while from the systematic extinctions also
-30-
follows that the atoms in the Y-CS.U.O.. structure occupy special
positions [1].
3.4.2. The crystal structures of β- and y-Cs^U.O
From the strong reflections of the X-ray pattern of γ-Cs-U.O „ it fol-
lows that the sixteen uranium atoms occupy position c(16) and the eight
cesium atoms position b(8) (space group Fd3m, origin at centre 3m).
Since the Cs-Cs distance is rather short (4.86 8) it is impossible that
an oxygen atom occupies position (i,i,i). More likely the oxygen atoms
occupy position f(48).
From five high-temperature Guinier films taken at 800° ± 20 C with
different exposure times, 22 intensities were measured with a Nonius
micro-densitometer. According to [73 the reflections were corrected with
a combined factor for absorption, oblique incidence and geometry correc-
tions Ν - 1.0+ l.O(l-cosX), in addition to the normal Lp-correction.
Testing the above described model for the y-Cs-U.Oj, structure,
oxygen atoms were located at position f(48) with χ • -0.051. The iso-
tropic temperature factors of uranium, cesium and oxygen were 2.5, 9.4
and 3.9 A respectively. As the (sin θ/λ) -range was rather small, how-2
ever, these values are not very reliable. The R -factor based on F was
7Z. Weights were estimated on basis of a comparison of the five exposures.
The atomic positions in the Y-CS„U,0.„ structure are summarized in
table 3.2.
Table 3.2.
Least-squares- coordinates of y-Cs U 0 .·
ATOM
U
Cs
0
x/a
0.000
0.375
0.125
σ (x/a)
-
y/b
0.000
0.375
0.125
o(y/b)
-
z/c
0.000
0.375
-0.051
σίζ/c)
.005
As B-Cs-U.O., is strongly related to the other two phases, as can be
seen from figure 3.1, its crystal structure will not greatly differ from
the a- and γ-structures. From films taken at 660° ± 20°C 77 intensities
covering 213 partly overlapping reflections with 2Θ < 60° were measured
with a micro-densitometer and corrected as described for the γ-phase in-
J
-31-
tensities.
Initial atomic positions were obtained from the γ-structure, but
adapted to the raonoclinic unit cell. Refinement of the uranium and2
cesium coordinates in space group P2./m led to an R-factor based on F
of 26.1%. Refinement of the structure in P2. led to an R-factor of 20.4Z.
According to Hamilton [5,6] the refinement in P2 gives the better des-
cription of the fi-Cs-U.O.- structure. Oxygen atoms could not be located
in a computed Fourier map. Due to the small range of (sin θ/λ) , an
overall temperature factor only has been refined: Β · 1.7 A • The
final position parameters of uranium and cesium are given in table 3.3.
Table 3.3.
Least-squares coordinates of ^~Cs2U4°i2 ^
pzi^'
.·•;ƒ>•••
ATOM
in
U2
U3
U4
Csl
Cs2
x/a
0.523
-0.018
0.264
0.738
-0.040
0.560
a(x/a)
0.013
0.012
0.014
0.012
0.015
0.014
y/b
0.000
0.052
0.234
0.264
0.811
0.274
a(y/b)
-
0.010
0.013
0.020
0.026
0.022
z/c
0.531
0.020
0.265
0.222
0.386
0.880
o(z/c)
0.009
0.007
0.009
0.009
0.010
0.008
Listings of the observed and calculated intensities of the X-ray
powder patterns of β- and γ-Cs-U.O _ are presented in appendices B7 and
B8.
3.5. Discussion
Most of the cesium uranates have crystal structures containing
(pseudo) hexagonal uranium layers like in U_0_ [8], as will be shown in
the next chapters. The three CSjU.O., crystal structures, however, are
related to the UO„ crystal structure. In figure 3.2 the y-Cs.U.O _
structure is drawn, together with the unit cell of U0_ t9]: the position
of the uranium atoms in Cs-U/O „ is similar to that of half of the
uranium atoms in U0_, while cesium atoms are substituted for the other
half. The introduction of cesium in the UO_ structure causes a decrease
in specific density from 10.952 for U0_ [10] to 6.613 for Y~C s
2
U4
Oi2·
As this compound was shown to be present in reactor fuel pins during
-32-
••••'·' 'ί.·';·.Ί
5.6iA
(O)
(a) and UO» (b) and an isolated structural
'0 ? has been built
Figure 3.2.
Unit oells of y-CsJJ.
motif in Ί-Cs^l^^ (β). The unit oell of i-
from 8 motifs such as given in figure a. From the figures b and a
the structural relation between U00 and y-Cs
0U.O _ is evident
(oxygen atoms are not drawn in these figures; small spheres designate
uranium, large spheres designate cesium).
•Wwiw^MBeim^s^-^na^Wer1 =Γ'
-33-
irradiation [chapter II], the swelling of the U0„ pellets during fission
can be readily understood.
The coordination of uranium by oxygen in γ-Cs-U.O „ is not the same
as in U0„. For ÜO„ an eightfold coordination with V-0 » 2.37 X has been
reported [9], whereas in Y-CS.U.O.J a deformed octahedral coordination
with U-0 * 2.07(2) has been found. Each cesium atom in y-CsJ}.0.„ is
coordinated by six oxygen atoms at 3.64(6) A and twelve oxygen atoms at
4.06(1) A. Since these cesium-oxygen distances are rather large Cl] the
large temperature factor of cesium probably indicates static disorder
or oscillation of the cesium atoms in the structure.
The differences between the uranium and cesium positions in the
Cs„U,O.„ crystal structures are rather small. The average Ü-U distances
for a-, 0- and y-Cs.U.O . are all in the range 3.8-4.0 X, while the
average Cs-Cs distances are in the range 4.8-4.9 8. Larger differences
may be found in the oxygen positions in the three structures, but neutron
investigations are necessary to affirm this hypothesis.
Finally there is the question whether CsAJ.O.* should be written asVI IV VI V
Cs„U„ U O.» or as Cs„U„ U„O.„. Measurement of the magnetic suscepti-
bility cf a-Cs.U.O.^j using a Gouy balance, showed a magnetic moment
which was not found in Cs_U_07 and Cs.U.O.,. Kemmler-Sack et al. [II]ill Z H I J ^ yj
have indicated that o-U_0o should be written as U_0,.U 0_. SimilarlyJO i D i
the uranium in a-Cs_U,0._ is assumed to be partly in the pentavalent andVI V
hexavalent state respectively, which implies that Cs„U_ U?0 „ is thecorrect notation for
References
[1] Internationa). Tables for X-ray Crystallography, Birmingham, (1962).
[2] M.J. Buerger, Vector Space, New York, (1959).
[3] D.T. Cromer and J.T. Waber, Acta Cryst., 2§_, 104 (1965).
[4] D.T. Cromer, Acta Cryst., _1£, 502 (1965).
C5] W.C. Hamilton, Acta Cryst., JJJ, 502 (1965).
[6] G.C. Ford and J.S. Rollett, Acta Cryst., A26, 162 (1970).
[7] W.H. Sas and P.M. de Wolff, Acta Cryst., 8, 104 (1965),
[8] 3.0. Loopstra, Acta Cryst., \7, 651 (1964).
<
-34-
t9] Thermodynamic and Transport Properties of Uranium Dioxide and
Related Phases, I.A.E.A. Technical reports series no. 39, Vienna,
(1965).
[10] E.H.P. Cordfunke, The Chemistry of Uranium, Amsterdam, (1969).
[II] S. Keranler-Sack, E. Stumpp, W. Riidorff and H. Erfurth, Z. anorg.
allg. Chem., 354, 287 (1967).
..I
••-Ui
. ί-'J.Y
-35-
CHAPTER IV. THE CRYSTAL STRUCTURES OF j3 AND CSjU Oj
fi
*)
4.1. Introduction
In sectipn 2.4.2 the reversible dissociation reaction
C s
2Vs3^
C s2
U4°12
+ *°2
has been discussed. Since in the foregoing chapter the crystal struc-
tures of Cs.U.O.» have been described, now a study of the Cs_U,0 „
structure is called for.
In chapter II it was found that the crystallinity of Cs„U,0 „
and CsJ 0 , powder samples increases upon heating above 600 - 700 C.
Furthermore Cs.U.O , appeared to have a homogeneity range when heated
above 650°C, whereas at 1000°C a solid solution is obtained with
0.375 < Cs/U < 0.5, enclosing the original compounds Cs.U 0 and
Cs„U O ,. In this chapter the crystal structures of Cs^U^O,., and Cs-U.O.,
will be described.
4.2. Experimental
Powder samples of Cs-U.O., were prepared as described in the second
chapter. For the structure determination of Cs.U-O., a sample with
Cs/U = 0.42 was used. After heating the sample in air at 760 C for
50 hours it was cooled down to room temperature very rapidly to avoid
decomposition of the one-phase component into Cs„U,0 and Cs?U.O.,
(Cs/U= 0.40),. as could be expected from earlier experiments (see figure
2.1).
Single crystals of Cs.U.O., were grown from molten cesium
chloride, on amorphous UO. in a gold boat. The gold boat had been placed
in a quartz tube, which was filled with pure oxygen. After one day
crystals had formed along a temperature gradient, the temperature
ranging from 950° to 900°C in the gold boat. The crystals had grown at
the outside of the boat, while the gold boat had been partly corroded by
some of the reagents. The plate-like, even scaly crystals were of a very
poor quality. Attempts to grow better crystals from different reaction
*) This chapter is a revised version of the paper by A.B. van Egmond,J. inorg. nucl. Chenu, in the press.
• -.-•4
,&?:;
?&3
&!
tftt*
•ν,Λ.Α»
-36-
circumstances and from other molten-salt techniques failed. In a number
of cases Cs,U,O._ crystals could be isolated from the reaction products.
A description of the crystal structure of this compound will be given in
the next chapter.
A.3. Crystal data and crystal structure of CsJJ.O.-
Guinier exposures of powder samples of Cs„U,O., could be indexed with
a C-face centered orthorhombic unit cell with a = 13.494(2) A,
b • 15.476(2) 8 and c « 7.911(2) 8, as follows from a least-squares
refinement on Q-values. The density of Cs„U,0.3 had been measured to be
6.8 (chapter II). Assuming 4 units Cs.U.O in the unit cell a density
5.73 can be calculated, which, however, is much lower than the experi-
mental value. With 5 molecules in the cell the density is calculated to
be 7.17, which in turn is too high.
Weissenberg exposures of some Cs^U.O., crystals showed that the unit
cell of Cs.U.C· has a c-axis five times the above mentioned c-axis. Thus
the true unit cell of Cs„ü,0 „ is a C-centered one with a = 13.494(2) 8,
b » 15.476(2) 8 and c » 39.56(1) 8. The systematic extinctions, deter-
mined from the Weissenberg exposures, correspond to the space groups
Cmcm, Cmc2 or C2cm [1]. From this cell a density 6.879 is calculated,
assuming 24 units CsJJ,O.~ in the cell, which is close to the experimental
value 6.8. Therefore the subcell of CsJJ.O „ contains 24/5 4.8 units
An irregular crystal of Cs-U.O.-, with a diameter < 0.05 mm, was
mounted on a PW1100 computer-controlled single-crystal X-ray diffracto-
meter. Using monobromated ΜοΚα-radiation a set of 360 subcell reflec-
tions with 2Θ < 50° was measured.
Since the unit cell consists of five subcells the symmetry of the
subcell is the same as the symmetry of the large unit cell. Therefore
the crystal structure of Cs-U 0 has been solved in the subcell with
spacegroup Cmcm. From a Patterson synthesis two pseudo hexagonal uranium
layers, consisting of 8 atoms each, could be constructed in the Cs.U.O.-
subcell, perpendicular to the y-axis. In a calculated Fourier synthesis
the remaining uranium atoms could also be located in position f(8). As
the subcell contains only 4x4.8 » 19.2 uranium-atom equivalents this
eightfold position is occupied for 40% on"y. In a difference Fourier
synthesis several unsharp maxima could bo detected. These positions were
τΜ: •Μ'.
II ·•.'··
•o - •
/" ;v/fir2
•4f^tf^^
-37-
filled with statistically distributed cesium atoms, their occupancy
roughly being based on the peak heights in the difference Fourier syn-
thesis. A least-squares refinement of this structure model, using cor-
rected scattering factors [2,3] resulted in an R-factor of 30%.
In the subcell each atom is a superposition of several atoms from
the true cell, with coordinates distributed around the subcell atomic
position according to the space group symmetry. This distribution of
coordinates might be represented by an anisotropic thermal refinement of
the subcell atoms, taking into account that these anisotropic parameters
have no thermal significance. In applying this procedure to the uranium
atoms in the Cs.U.O.. subcell the R-factor immediately dropped to 18.5%,
indicating that the constructed model is basically correct. Concerning
the reflection set there should be made two remarks: at first the
crystals of Cs.U,0._ were of a very poor quality, and secondly some sub-
cell reflections might have suffered from overlap with non-subcell reflec-
tions, because of the very large c-axis and the use of the molybdenum
radiation. Therefore no refinement calculations have been made in other
space groups.
Table 4.1.
Least-squares coordinates of CsJJ.O.- (subcell Cmcm).
Atom
Ul 4c
U2 4c
U3 8d
U4 8f
Csl 4b
Cs2 4c
Cs3 8g
Cs4 8e
x/a
0.000
0.000
0.250
0.000
0.000
0.500
0.314
0.328
σ(x/a)
-
-
-
-
-
-
0.003
0.005
y/b
0.202
0.211
0.250
0.388
0.500
0.591
0.490
0.500
0(y/b)
0.00!
0.002
-
0.001
-
0.010
0.004
-
z/c
0.250
0.750
0.000
0.024
0.000
0.250
0.250
0.000
a(z/c)
-
-
-
0.001
-
-
-
-
occupancy
1.00
i.00
1.00
0.40
0.40
0.20
0.60
0.30
*)F — Fobs calc
| / £ Fobs*
•]•"·. i-
-38-
The final coordinates of the Cs.U.O.» structure, resulting from the
least-squares refinement of the subcell coordinates in space group Cmcm,
have been summarized in table 4.1. A list of observed and calculated
structure factors has been included in appendix A2, whereas the powder
X-ray pattern is given in appendix C3.
4.4. Crystal data and crystal structure of Cs_U,016
A powder sample of Cs.U.O.,, mixed with starch to avoid preferred orien-
tation, was mounted on a Philips PW1150 powder X-ray diffractometer. It
was step-scanned from 8.00° up to 65.00 2Θ in steps of 0.02 , using Ni-
filtered CuKct radiation. The measured profile was corrected with an
estimated background. The pattern could be indexed with a monoclinic
C-centered unit cell, with a - 13.465(2) 8, b - 15.561(2) 8,
c « 15.928(4) X and β - 92.78(1)°, as followed from a least-squares
refinement on Q-values. The systematic extinctions correspond to the
space groups C2/c and Co.
Omitting some weak reflections, a subcell was found with c' = £c =
7.964(2) A. Due to the symmetry of the large cell the space group of the
subcell is C2/m or Cm respectively. A calculated density of 6.822 cor-
responds to 8 molecules Cs_UcO,, in the true cell; and therefore 4 in
ζ 5 IDthe subcell.
It was decided to solve the crystal structure of Cs.U.O,, based onL D ID
the subcell with space group C2/m. The X-ray profile was integrated to
95 intensities covering 314 partly overlapping reflections with 26-angles
up to 65 . Since Guinier films of Cs.UcO., and Cs„U.O,_ resemble each
i D Ιο Ζ 4 13
other very much, hexagonal uranium layers could be constructed in the
Cs_U.O subcell. A difference Fourier synthesis revealed several weak
maxima, indicating a statistical distribution of the cesium atoms inthe subcell. After inclusion of the cesium atoms the R-factor on F'
dropped to 14.8%, indicating the model is fundamentally correct. An
attempt to refine the structure in the unit cell with c » 15.928 X and
space group C2/c failed to show more distinct positions for the cesium
atoms. In table 4.2 the final coordinates of the subcell refinement are
listed. The structure model has not been refined in space group Cm.
*)
7)R Γ Ι Σ η F
2, - 7. η F
2 , Ι / Ζ η F
21 obs calc ι obs
«tj&'f ••;•'**
i-m'
. 'ι
-39-
A listing of the observed and calculated intensities in the powder
pattern is presented in appendix B4.
table 4.2.
Least-squares coordinates of CsJJJ)-~ (subaell C2/m).
Atom
Ul 8j
U2 4e
U3 4f
U4 4g
Csl 4i
Cs2 4i
Cs3 2d
Cs4 4i
x/a
0.013
0.250
0.250
0.000
0.223
0.796
0.500
0.147
a(x/a)
0.002
-
-
-
0.010
0.007
-
0.008
y/b
0.205
0.250
0.250
0.389
0.000
0.000
0.000
0.000
σ(y/b)
0.001
-
-
0.002
-
-
-
-
z/c
0.270
0.000
0.500
0.000
0.196
0.223
0.500
0.506
CT(Z/C)
0.004
-
-
-
0.012
0.014
-
0.016
occupancy
1.00
1.00
1.00
1.00
0.50
0.50
1.00
0.50
! \
Μ-
4.5. Discussion
From this investigation, it appears that the structures of Cs_U,0 _
and Cs-ILO., are very similar. The skeleton of the uranium atoms in both
compounds has been reproduced in figure 4.1. In the structures of
Cs^i^O.- and Cs^U-O., the pseudo hexagonal uranium layers are linked by
bridges consisting of two uranium atoms. In CS.U..0 , these bridges areZ j Jo
found more frequently than in Cs„U,O.,. Also the positions of the cesium
atoms are not the same in both structures as follows from a comparison
of the tables 4.1 and 4.2. Since oxygen atoms have not been located in
the structures, conclusions concerning the coordinations could not be
drawn. However, in the last chapter the oxygen positions in the double
bridges will be discussed. The double bridges linking the hexagonal
uranium layers have not been found before, whereas hexagonal uranium
layers have been found in other alkali uranates and uranium oxides
[4-10].
Due to the structural resemblance of Cs.,U.O,_ and CsoU.0,, it is
/ 4 IJ ί ο Ιο
interesting to examine the corresponding part of the phase diagram of
the Cs-U-0 system in air (figure 2.1) more closely. The question arises
how the solid solution, which is formed above !000°C between Cs.U.O.. Β&-
y . % ; » • • • 'iTafrtf;jfr •·=-'•'• * » ίί •
- 4 0 -
• } ' r
I ••·•"•
ι •'.
V'
Figure 4.1.
Uranium positions in the subeells of CsJ1.0.~ and CsJJ,-O-„.ύ 4 J.O ώ O ΙΌ
The blaak spheres form the double uranium bridges.
- 4 y '-I -
' " ' . ••'" #••:'
\ ?'%•
' -· - • ' ' ••
';-M
mΜ
W
and Cs„ü,0.,, can be limited by an orthorhombic compound Cs„U,O.„ and a
monoclinic compound Cs-ULO.,. Reinvestigation of the high-temperature
Guinier films has yielded the following more, precise picture of the
phase relationships between Cs.U.O _ and Cs.U.O.,. At room temperature
Cs-U.O.» crystallizes in an orthorhombic cell with a c-axis of about 40&,
whereas Cs_U_O,, has a monoclinic unit cell with a c-axis of about 16A./ j ίο
At 600 C the Cs-U-O., c-axis is halved, the unit cell remaining mono-
clinic. Above 1000 C the CS2Ö,O.„ c-axis is changed to an axis of about
8A, whereas the crystal system becomes slightly monoclinic, as follows
from the splitting of the X-ray lines on a high-teirperature Guinier film.
In this smaller cell the structures are statistically disordered. Now a
solid solution can exist between Cs_U.O,_ and Cs_U_O,,, as shown in: 4 Π <i 3 ID
figure 4.2. In this monoclinic solution the monoclinic β-angle is a
- 4 1 -
. , {··Ι! .
1100. .
900.
700.
500.
•.it
0.4 05
Cs/U ratio
Figure 4.2.
Part of the phase diagram of the pseudo-binary Cs-U-0
system in air. The dotted lines represent metastable
equilibria in open atmosphere (p(CsJ3)=0). The figure
should be compared to figure 2.1.
function of the temperature and the Cs/U ratio. However, at 900°C all
samples decompose into Cs.11,0 „ and U_0„ after long heating times.
Therefore some lines in figure 4.2 have been dotted because they re-
present metastable equilibria (compare to figure 2.1).
As the CSjU.O.- crystals, used in this study were allowed to cool down
slowly the orthorhombic 40A-phase was obtained. Upon cooling down rapidly
the Cs.UeO.g sample the monoclinic 8A cell could be quenched in (apart
from some very weak lines indicating a transformation to the 16A* cell),
The unsharp patterns of both compounds below 600 C can be explained by
assuming that the long range order cannot be obtained perfectly at reac-
Γ.-·
....
• , - r * •·*•• ;'«r-:-.-""
ifΙ - •
• '& *>". Γ
-42-
tion temperatures of 500 - 600°C. This fact also accounts for the poor
quality of the Cs U,0 _ crystals.
References
[1] International Tables for X-ray Crystallography, Birmingham, (1962).
[2] D.T. Cromer and J.T. Waber, Acta Cryst., U±, 104 (1965).
[3] D.T. Cromer, Acta Cryst., Jj}, 17 (1965).
[4] L.M. Kovba, Radiokhimiya, Jjl, 727 (1972). A
C5] L.M. Kovba, Zh. Strukt. Khimii, J2· 2 5 6
0972).
[6] L.M. Kovba and V.I. Trunova, Radiokhimiya, JJ3» 7 7 3
(1971).
[7] J.S. Anderson, Chimia, 2!3, 438 (1969).
[8] B.O. Loopstra, Acta Cryst., J_7, 651 (1964).
[91 P.C. Debets, Acta Cryst., 2^, 589 (1966).
[10] C. Greaves and B.E.F. Fender, Acta Cryst., B28, 3609 (1972).
.·•?/ •
. I: '•·
-43-
CHAPTER V. THE CRYSTAL STRUCTURES OF
AND Cs2UO
4
*)
5.1. Introduction
In chapter IV the crystal structures of two cesium uranates, in which
uranium is in the hexavalent state, were described. In this chapter the
crystal structures of Cs^OjO^, C s ^ O ^ , C s ^ g O ^ and Cs^O^ are
presented. The only cesium-uranate(VI) crystal structure left is that of
Cs-U-O-,, which will be dealt with in chapter VI.
5.2. Experimental
Powder samples of Cs-UO,, Cs_U_O__ and Cs_U ,0,, were prepared as des-
cribed in chapter II. The hygroscopic samples of Cs.UO, were handled in
a dry box. The samples of Cs_U,O„„ and Cs_U,,O., always contained im-ί I it i I 3 4D
purities like other uranates and U,0_.
Single crystals of Cs.UJJ were made in a number of different ways:
- reaction of amorphous UO- with molten CsCl yielded yellow transparant
Cs,U,0 ? crystals after 120 hours of heating in air at 700°C in a
gold boat;
- the same crystal needles were grown in a reaction of amorphous U0_
with CSjSO in a platinum cup after 50 hours at 1100°C in air (in the
same experiment Cs„U,O.„ crystals were formed, as described in
section 3.2);
- several experiments starting with CsCl or Cs_S0, and UO, or cesium
uranates like Cs.U 0 , Cs„U5O ,, etc. in molten quartz tubes yielded
Cs.U 0 crystals after heating at different temperatures (700-1000°C)
for several days.
Attempts to grind the needle-shaped Cs.U.O.- crystals in a crystal
spherizer yielded strongly ellipsoidal crystals.
5.3. Single-crystal analysis and structure of Cs,U5O
)7
A single crystal of Cs.UjO.- with a diameter = 0.1 mm was mounted on the
sample support of a Philips computer-controlled single-crystal X-ray
*) This chapter is a revised version of the paper, by A.B. van Egmond,J. inorg. nucl. Chem., in the press.
ι • . · ' • .
J*·.
il
-il •
-44-
diffTactometer. The crystal showed an orthorhombic unit cell with cell
dimensions a - 18.776(4) X, b - 7.070(1) 8 and c - 14.958(3) 8 (these
cell parameters were refined from a powder diffractogram of Cs,U_0I 7);
the systematic extinctions (Okl absent for k=2n+l, hOl absent for
l = 2n+l and hkO absent for h+k=2n+l) correspond to space group Pbcn
uniquely [J], Assuming four Cs.U.O.- units in the cell, the calculated
density is 6.669, close to the measured value 6.62.
In the range 6.00 < 2Θ < 50.00 1723 reflections were measured
with monochromated MoKct radiation, 1553 reflections being greater than
three times the counting standard deviation of the intensity. An ab-
sorption correction was lot applied to the reflection set.
The positions of the uranium and cesium atoms were found using the
direct-method programs SINGEN and PHASE of the X-RAY SYSTEM [2]. After
subsequent refinement of the position parameters and three-dimensional
Fourier calculations all oxygen atoms could be located in the Cs.U O._
crystal structure. The weighted refinement of position parameters,
thermal parameters and scale factor was based on F, with weights from
statistics; scattering factors were taken from Cromer and Waber [3] and
i
Table 5.1
Least-squares coordinates of Cs.U,0-„.
Atom
UI
U2
U3
Csl
Cs2
01
02
03
04
05
06
07
08
09
4c
8d
8d
8d
8d
4c
8d
8d
8d
8d
8d
8d
8d
8d
x/a
0.5000
0.1035
0.3118
0.3072
0.0548
0.000
0.079
0.132
0.212
0.381
0.466
0.322
0.296
0.078
σ
0
0
0
0
0
0
0
0
0
0
0
0
(x/a)
-
.0001
.0001
.0002
.0002
-
.002
.003
.002
.002
002
.002
003
002
0
0
0
0
0
-0
0
0
0
0
0
0
-0
0
y/b
.1330
.0781
.0267
.3004
.2718
.035
.078
.109
.204
.257
.145
.051
.015
.388
a(y/b)
0
0
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0004
0003
0003
0007
0007
008
009
009
005
005
007
008
010
006
0
0
0
0
0
0
0
0
0
0
0
0
0
0
z/c
.2500
.2082
.1875
.4458
.4705
.250
.113
.304
.174
.203
.356
.081
.309
217
σ
0
0
0
0
0
0
0.
0.
0.
0.
0.
0.
(z/c)
-
.0002
.0002
.0003
.0003
-
007
006
004
004
004
008
002
004
FV-V
·..*, *ΐ';' '$""J
•J 'i, ,.
! ! ' · " ' . •
-45-
corrected for anomalous dispersion with Af', according to Cromer [4].
In the last stages of the refinement uranium and cesium atoms were allowed
to vibrate anisotropically, but, since an absorption correction had been
omitted, some temperature factors were non-positive definite. The final
R -factor was 21.3% ; the position parameters resulting from the last
refinement cycle have been listed in table 5.1. In figure 5.1 the coordi-
nation of the uranium atoms by oxygen has been drawn, whereas in figure
5.2 an impression of the crystal structure of Cs,U,O._ is given: the
layer composition of Cs.U.O.y has been very clearly expressed. The
relevant interatomic distances and angles in the uranyl groups of the
Cs.U.O.- crystal structure, as calculated with aid of the BONDLA routine
of the X-RAY SYSTEM [2], are summarized in tabia 5.2. A list of observed
and calculated structure factors is presented in appendix Al, whereas
the powder pattern is listed in appendix C2.
y*
Figure S.I.
The oxygen coordination of uranium in the pseudo hexagonal uranium
layer (uranyl oxygen atoms are not shown; the angles are in degrees,
accurate to = 2°; distances are in X, accurate to 0.05 %.).
*)*«lc - 'ob.
cobs
I., "
Stereoscopic view of Cs.U/)„ along the b-axis (large spheres
designate cesium atoms, small spheres deszgnate uramum atoms)
Table 5.2.
Interatomic distances and angles in uranyl groups of Cs,UrO-„
atoms
UI-06
U2-02
U2-03
Ü3-07
U3-03
distances
1.71(6)
1.50(9)
1.54(8)
1.61(11)
1.87(4)
atoms
06-UI-06
02-U2-03
07-U3-08
angle
174 °(3)
171 °(3)
176 °(3)
5.4. The crystal structure of Cs2U7O22
As stated in chapter II Cs2U_O2„ is isostructural with the corresponding
rubidium and potassium uranate, RbjU-O-n [5] and KjUyOu» C63 respectively.
A sample of Cs2U7O22 was mounted on a PW1150 powder diffractameter,
after mixing the sample with starch to avoid preferred orientation. The
diffractogram of the sample was step-scanned from 8.00° to 65.00° 2Θ in
steps of 0.02° using Ni-filtered CuKa radiation. The X-ray spectrum
fej
, •l'ïji-'.ï*-*.'1'
ν
-47-
could be indexed on an orthorhombic unit cell with a « 6.949(1) A,
b = 19.711(2) X and c = 7.3955(8) 8. The space group is Pbam or Pba2 as
follows from the systematic extinctions (Okl absent for k*2n+l,
hOl absent for h«2n+l). With two Cs.UyO». units in the cell a density
of 7.488 is calculated.
Since Cs.U-O», samples always contained small amounts of Cs-U-O.,
and Cs.U.-O,, the profiles of the latter compounds were appropriately
subtracted from the Cs.U-O-» profile. Thereafter the diffractogram of
Cs-U_O._ was integrated with a computer program to yield 92 intensities
covering 219 reflections with 2Θ up to 67°.
A model, based on Kovba's work on K_U7O_. [6], without the oxygen
*)atoms, was refined in space group Pbam to an R-factor of 20%. On changing
the space group to Pba2 and including uranyl oxygen atoms, the R-factor
did not improve. The coordinates, resulting from the last refinement
cycle in space group Pbam are listed in table 5.3; the layer structure
of Cs„U_O - is shown in figure 5.3. Appendix B5 contains the observed
and calculated X-ray pattern of Cs.U-O.-·
Figure 5.3,
Stereoscopic view of CsJJ„0^s along the ,^-tixis (large spheres
designate cesium atoms, small spheres ·'• "innate uranium atoms).
*>R - Σ Ι Σ η - Ε η / ζ η
Pi-o ,· „Γ f.;- j ^
"'«'<·'•* *".,'
-48-
5.3.
Least-squaves coordinates of
Atom
Ul 2a
U2 4g
U3 4g
U4 4h
Csl Ah
x/a
0.000
-0.021
-0.040
0.230
0.174
o(x/a)
-
0.006
0.005
0.005
0.008
y/b
0.000
0.218
0.408
0.039
0.309
o(y/b)
-
0.002
0.002
0.002
0.003
z/c
0.000
0.000
0.000
0.500
0.500
o(z/c)
-
-
-
-
-
ί * •'
5.5. The crystal structure of Cs-U.,.0,,
The diffractogram of the cesium uranate with the lowest Cs/U ratio was
obtained as described for C s ^ O ^ . The X-ray spectrum could be indexed
on an orthorhombic unit cell with a » 14.686(3) A°, b » 13.422(2) X and
c = 19.752(3) 8; the space group is Cmca or C2ca (hkl absent for h+k=2n+l,
hOl absent for l = 2n+l, and hkO absent for k»2n+l). The profile was
corrected for small amounts of <*-U3Og and Cs
2U
7O
2 2 > whereafter the
profile was integrated to 68 non-zero intensities, covering 317 reflec-
tions with 2Θ < 61°.
Since the b-axis in Cs^^O^g is twice the a-axis in CsJ 0», and
the c-axis in C S J U J J O ^ is equal to the b-axis in C s
2
U7°22'
t h e P
resence
of hexagonal layers parallel to the bc-plane in Cs-U.-O,, was obvious.
After some Fourier calculations in space group Cmca only 68 "heavy"
atoms could be found in the unit cell, which together with the rough
composition ratio 0.10 < Cs/U < Q.15 (see chapter II) leads to the
formula CSgUj
sity is calculated to be 7.800.
In further refinements in space group Cmca an indication which
atoms are cesium and which are uranium could not be found for the inter-
layer atoms. The final R-factor on this rather poor model was 28%. The
coordinates of the atoms are listed in table 5.4, the structure being
drawn in figure 5.4. A list of observed and calculated intensities is
presented in appendix B6.
. Assuming 4 units Cs.U 50,- in the unit cell the den-
*)Fobs
Fcalc
jni^uè
Atom
UI
U2
U3
U4
U5/Csl
U6/Cs2
U7/Cs3
-49-
Table 5.4.
Least-squares coordinates of Cs2Uib°46
Τ
16g
I6g
8e
x/a
0.235
0.285
0.250
4a I 0.000
8f
8£
8f
o(x/a) ι y/b
0.005
0.004
0.
0.
0.
000
000
000
0.127
0.389
0.101
0.000
0.288
-0.037
0.169
0.003
0.004 I
o(y/b) z/c |o(z/c)
0.005 I 0.066
0.005 i 0.149
0.007 0.250
0.000
0.126
0.206
0.007
0.007
0.007 0.392
0,006
0.006
0.006
occupancy
1.0
1.0 j
1.0 J
1.0 I
0.67/0.33 I
0.67/0.33
0.67/0.33
,1 Λ
Figure 5.4.
Stereoscopic view of CspU .O.g along the b-axis (large spheres
designate statistically distributed uranium/aesiion atoms 2:1,
small spheres designate uranium atoms).
5.6. The crystal structure of CsJJO,
Since Cs„UO, is very hygroscopic, its X-ray diffractogram had to be
recorded from a sample in plastic foil. The sample had been mixed with a
small amount of α-quartz for calibration purposes. The unit cell of
Cs2UO, is tetragonal with parameters a » 4.3917(6) X and c » 14.804(3) 8.
The symmetry corresponds to the space group 14/nmrn, as was found pre-·
viously by other investigators [7-9].
ft
a
-50-
From the diffractogram 37 intensities were integrated by hand,
covering 44 reflections up to 80 2Θ. A model, based on the results of
Kovoa [7], was refined with these intensities, omitting two reflections
which suffered from overlap with α-quartz reflections. The resulting
cesium parameter z/c, as listed in table 5.5, is in good agreement with
Kovba's results, but the uranyl distance U-0„ is too large: 2.34(1) A.
2The final R-factor on F was !4.3%. A stereoscopic impression of the
structure is given in figure 5.5; the final least-squares coordinates
have been summarized in table 5.5. In appendix Bl a list of observed and
calculated structure factors is given.
l·
Table 5.5.
Least-squares coordinates of CsJJO..
ΓAtom
U 2a
Cs 4e
01 4c
02 4e
x/a
0.000
0.000
0.000
0.000
o(x/a) y/b o(y/b)
0.000
0.000
0.500
0.000
z/c
0.000
0.345
0.000
0.160
o(z/c) I
0.005
0.010
Figure 5.S.
Steveosaopia view of CsJUO, along the b-axis (large spheres
designate oesium atoms, small spheres designate uranium atoms).
Oα
JV * ^ " P j ^ ' " ' '"', Ϊ-U^l ^
wsMiwtóïlfeSp'
ÉÜÉ
•:'<-ï&
-51-
5.7. Discussion
In case of Cs„UlCO,, the X-ray analysis has been a tool to define theΖ 15 At)
composition of the compound. The structure model itself is rather poor,
due to impurities in the sample and considerable overlap in the pattern.
The same facts affect the structure model of Cs Ü 0 .. In both compounds
mixed cesium-uranyl layers have been shown next to (pseudo) he:.agonal
uranium layers. Therefore these compounds might be called cesium uranyl
uranates, in agreement with the nomenclature of Kovba [6]. In Cs„U,cO,,
the exact positions of the cesium atoms could not be established. One
could speculate that the cesium atoms in Cs.U .0,, cluster together in
short zig-zag chains like in Cs-U-O-o» rather than all cesium atoms being
separated by uranyl groups. In this way a kind of short-range order can
be obtained.
Since the crystal structure of Cs_U0, contains tetragonal uranium
layers it is related to Rb.UO, and K„UO, [9], Together with these alkali
uranates Cs„U0, belongs to the K.NiF, structure class [10]. As the in-
tensities of Cs_UO, suffered probably from preferred orientation, the
results of this structure study are net as accurate as the results of
Kovba et al. [6].
The crystal structure of Cs.U 0 - contains endless layers of pseudo
hexagonally arranged cesium atoms only between the likewise hexagonally
arranged uranium layers. In the uranium layers the uranium atoms are
coordinated by six of seven oxygen atoms, two oxygen atoms always be-2+
longing to the uranyl group U0„ . The oxygen atoms in the uranyl groups
link the cesium layers and the uranium layers. The uranium-oxygen dis-
tances in the uranyl groups vary from 1.50 A to 1.87 A. In other crystal
structures uranium-oxygen distances in uranyl groups are reported to
range from 1.70 8 to 1.95 A [6-8,11-13]. In the hexagonal layers uranium
is coordinated by four or five oxygen atoms, the uranium-oxygen dis-
tances ranging from 2.09 X to 2.51 X.as has been shown in figure 5.1.
Although all distances may be somewhat affected by the omission of an
absorption correction, the general coordination of the uranium atoms can
be supposed to be correct.
References
[1] International Tables for X-ray Crystallography, Birmingham, (1962),
[2] See chapter I, reference [23]. ft
• J
-52-
[3] D.T. Cromer and J.T. Waber, Acta Cryst., JjB, 104 (1965).
[4] D.T. Cromer, Acta Cryst., JJJ, 17 (1965).
[5] L.M. Kovba and V.I. Trunova, Radiokhimiya, JJ, 773 (1971).
[6] L.M. Kovba, Zh. Struct. Khimii, \3_, 256 (1972).
[7] L.M. Kovba, I.A. Murav'eva and A.S. Orlova, Radiokhimiya, \b_,
648 (1974).
[8] H.R. Hoekstra, J. inorg. nucl. Chem., 27_t 801 (1965).
Γ9] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn,
Russ. J. Phys. Chem., jSS, 275 (1961).
[10] D. Balz and K. Pliefh, Z. Electrochemie, 59, 545 (1955).
Γ Ml L.M. Kovba, Radiokhimiya, Jjl, 727 (1972).
[i2] L.M. Kovba, Radiokhimiya, J2» 3 0 9
('971)
Γ13] Κ. Ohwada, Spectrochimica Acta, 4A, 595 (1967).
f!
·•:*.'*
-53-
CHAPTER VI. THE CRYSTAL STRUCTURES OF Cs^O., *^
6.1. Introduction
In chapter II it has been reported that Cs U_O_ exhibits three different
phases. Two phases, called a- and B~Cs U„0_, have a reversible phase
transition at 300 C. A third diuranate, the γ-phase, was found to be
metastable with respect to the β-form at 600°-800°C.
Recently y-Cs„U„O7 has been described by Kovba et al. [1], its unit cell
belonging to the hexagonal crystal class. The crystal structure was
reported to consist of hexagonal uranium layers and cesium layers,
linked by the uranyl oxygen atoms, whereas the remaining oxygen atoms
were statistically distributed in the hexagonal uranium layer.
In this chapter the cryst.nl structures of the three cesiunrdiuranate
phases are reported, completing the description of the cesiunr-uranate(VI)
crystal structures.
6.2. Experimental
Powder samples of the diuranates were synthesized as described in
chapter II. Pure samples of β-Cs.U-O- were obtained on heating mixtures
of amorphous UO. and cesium carbonate at 600 C. This phase changes
into a-Cs„U„0_ when the temperature is kept below 300 C for several
days. The change in the X-ray pattern due to the phase transition is
shown in figure 6.I indicating only a small structural change. Upon
rapid cooling from 600 C down to room temperature the crystal structure
of B-Cs„U-O_ is quenched.
The X-ray pattern of y-Cs-U-O- always showed broad vague lines.
The γ-phase also appeared to contain small amounts of (5-Cs?U.0_. The
experimental circumstances concerning the γ-diuranate formation have
been described extensively in chapter II.
6.3. The X-ray analysis of ct-Cs-UjO-
A powder sample of α-Cs.U.O-, mixed with starch, was mounted on a
rotating sample support of a Philips powder diffractometer. The sample
*)This chapter is a revised and extended version of the paper byA.B. van Egmond, J. inorg. nucl. Chem., in the press.
:·$&$$•& S I
MfiSP
200-
400-
600-
•%',•',:
^;-^
'20
Figure 6.Ί.
High temperature film of CsJJJD , showing the X-ray patterns of the
X-ray patterns of the two monoolinia diuravate. phases and Cs.UJJ.^.
ο ->• Β transition at SOO°C. β •* Cs Uβ.- decomposition at - 650°C.
(Guinier film taken in air, monochromated CuKa radiation).
-55-
contained small amounts of Cs.U.O.-. Its X-ray pattern was step-scanned
from 10.00° to 78.74° 2Θ in steps of 0.02°. The pattern could be indexed
with a monoclinic C-face centered unit cell with a « 14.528(3) A,
b = 4.2638(7) X, c = 7.605(1) X and 6 = 112.93°. Thereafter the profile
could be integrated to 56 intensities covering 153 reflections. The space
group of the α-diuranate is C2/m, Cm or C2 (hkl absent for h+k = 2n+l).
The b/c-ratio of the unit cell allows a nearly ideal hexagonal
uranyl oxygen layer parallel to the bc-face of the cell. These
(pseudo) hexagonal layers have been established in a number of cesium
uranates [chapter IV,V], However, the strong 001 reflection indicates
a structure with sheets parallel to the centered ab-face. Nevertheless
it was impossible to describe the structure as a (pseudo) hexagonal
uranium network parallel to the ab-face due to the centering.
A three-dimensional Patterson synthesis, based on 23 reflections,
showed the heaviest peak in position 0,i,i. Assuming this peak to be
the U-U vector, the Patterson function favours the model with layers
parallel to the bc-face. Several configurations according to this model,
based on uranium and cesium atoms only, did not result in a satisfying
agreement of (summed) observed and calculated squared structure factors.
Therefore an electron-microscope study was undertaken to obtain
more information about the unit cell. From several powder particles of
the diuranate electron diffraction images could be obtained confirming
the proposed unit cell. Two examples of the electron diffraction images
are shown in figure 6.2. In addition a neutron diffractogram was in
accordance with the indexing of the X-ray pattern (see section 6.6).
The heaviest Patterson peak could also be regarded to be the super-
position of all U-Cs vectors in the unit cell, which allows a non-
(pseudo)hexagonal uranium layer parallel to the ab-face. A model based
on uranium and cesium atoms only in C2/m rapidly resulted in an R-factor
of 11%. A difference Fourier synthesis revealed that all oxygen atoms
are situated at reasonable distances from the uranium atoms. In further
computations the 002 reflection appeared to have a very heavy weight in
the least-squares refinement. After neglecting the 002 reflection the
R-factor dropped to 5.8%. Scattering factors were taken from Cromer and
Waber [2] and corrected for anomalous dispersion with Af', taken from
Cromer [3D. The overall isotropic temperature factor was refined to
*} R - Σ Ι η F
2,1
obs- η F
2 . | / Σ η F
2,
calc ' obs
o.-·»;..
' 'i
-56-
• ί· h'
Ιf.
•¥<
Figure 6.2.
Electron diffraction images of CsJJJ}^. Because of the tem-
y< rn'ni'i' increase nf the sampla by electron absorption it is >u>
clear to which phase (a or B) the diffraction images refer.
The reciprocal axes have been indicated. After calibration
with Mo, Al and Nb the following cell constants could be
calculated for· Cs^UJ) : a = 14.b %, b = 4.3 %, a = 7.6 %. and
β - lib0. Deviations from exact cell constants are caused by
non-proper alignment of the crystallites. Electron diffraction
images were taken using a Philips EM Z01, operated at 80 kV.
\
"3 ,
; !--Λ '.
-57-
» , . . _ •· '•
ϊ· ν
0.72(9) Χ , which is reasonably reliable considering the maximum
(sin 6/A)2-value 0.167 S~
2.
In the final model the 0,-atom is located in a position which is
only half occupied, as is shown in figure 6.3a. This fact might be
caused by the choice of space group C2/m instead of C2. Since the
R-factor was low, indicating that the uranium and cesium positions are
reliable and the structure factors are rather insensitive for a change
in the oxygen atom positions, the structure model has not been refined
in other space groups. The coordinates of the final refinement have been
listed in table 6.1, whereas table 6.2 contains the most relevant data
of the coordination of uranium by oxygen. A steroscopic view of the
structure has been given in figure 6.4. The observed and calculated
X-ray pattern of ct-Cs U 0? is given in appendix B2.
Table 6.1.
Least-squares coordinates of a-Cs J) J)„ from the X-ray data.
Atom
U 4i
Cs 4i
01 4i
02 4i
03 4i
04 4g
x/a
0.1465
0.3909
0.204
0.401
0.318
0.000
σ(x/a)
0.0006
0.0009
0.005
0.006
0.005
-
y/b
0.000
0.000
0.000
0.500
0.000
0.241
o(y/b)
-
-
-
-
-
0.030
z/c
-0.007
0.562
0.25
0.27
-0.01
0.00
o(z/c)
0.002
0.002
0.01
0.01
0.01
-
occupancy
1.0
1.0
1.0
1.0
1.0
0.5
Table 6.2.
Uranium-oxygen distances in a- and &-CsJJ„0„ from X-ray and neutron data.
Distance
U-01
U-02
U-03
U-03'
U-04
a~
C s2
U2°7
X-ray
1.81(9)
1.87(10)
2.49(5)
2.19(5)
2.38(7)
neutron
1.81(2)
1.88(2)
2.34(2)
2.26(2)
2.21(2)
e-cs2u
2o
7
X-ray
1.87(7)
1.95(6)
2.24(4)
2.30(4)
2.15(1)
-58-
Fignre 6.3.
Uranium layers in a-Cs^U^O^ (a) and $-Cs£)£ (b), projected on the
ab-face. Small oirales designate uranium atoms; large airoles designate
oxygen atoms; uranyl oxygen atoms are not shown. For a-Cs0U'Or, the6 2 7
neutron diffraction data have been used, the O.-atom being statistically
distributed over tuo positions.
•
Figure 6.4.
Stereoscopic view of a-Cs„UJ)? along the b-axis (large spheres designate
cesium atoms, small spheres designate uranium atoms; oxygen atoms are
not shown).
*l'"
Kfm
^m - •:· :.
Η --^• α
I;·:ί!
·*.\\\
-59-
'•\f,.''•''.
te
6.4. The X-ray analysis of e-Cs-U^
Α sample of U-Cs-U-Oy was heated at 600°C for 16 hours. Guinier films
after quenching the structure did not show any impurities.
The X-ray pattern of %-CsJSJ)-. was recorded as described for the
α-phase. The pattern could be indexed with a C-face centered monoclinic
cell with a - 14.516(2) X, b - 4.3199(6) 8, c - 7.465(1) % and
β - 113.78° (1). Up to 62.00° 2Θ its profile was integrated to 55 in-
tensities covering 82 zero, non-zero and overlapping reflections. The
space group of β-Cs-U-O- was also assumed to be C2/m (hkl absent for
h+k - 2n+l).
For the structure refinement the uranium and cesium coordinates of
the α-phase were taken as initial parameters. Oxygen atoms could be
located in a difference Fourier synthesis. Again the 002 reflection was
omitted from the refinement whereafter the R-factor dropped to 5.3%. The
isotropic temperature factor Β » 0.15(15) A is less reliable compared
to the temperature factor of a-Cs_Uo0,, due to the smaller (sin θ/λ) -
range. Final coordinates of β-Cs.U-O- are listed in table 6.3. The
uranium coordination by oxygen is summarized in table 6.2, and shown in
figure 6.3b. A list of observed and calculated structure factors is pre-
sented in appendix B3.
Table 6.3.
Least-squares coordinates of B-(7s„ü„CL from the X-ray data.
Atom
U 4i
Cs 4i
01 4i
02 4i
03 4i
04 2a
x/a
0.1474
0.3978
0.206
0.399
0.294
0.000
σ(x/a)
0.0006
0.0007
0.004
0.004
0.003
-
y/b
0.000
0.000
0.000
0.500
0.000
0.000
a(y/b)
-
-
-
-
-
-
z/c
-0.004
0.584
0.27
0.29
-0.04
0.00
a(z/c)
0.001
0.001
0.01
0.01
0.01
-
occupancy
1.0
1.0
1.0
1.0
1.0
1.0
Α»^%ί*.
'\\ ••
· • • » > . •
• · " £r'.
-60-
6.5. The X-ray analysis of Y - C S2U
2O
7
Notwithstanding the experimental problems with the synthesis of hexagonal
Cs-U.O. a Guinier film (Ni-filtered CuKct radiation) could be obtained,
with was clear enough to calculate the unit cell parameters. A least-
squares refinement of 25 Q-values yielded a hexagonal cell with
a » 4.108(1) & and c * i4.646(5) 8. These dimensions agree reasonably
with the results of Kovba et al., who reported a * 4.106(3) X and
c - 14.58(2) X [1], Possible space groups for the hexagonal γ-diuranate
are P6-/mmc, PE2c and P6_mc (hh2fil absent for l»2n+l). The density,
calculated from the X-ray unit cell, is 6.624(5) assuming one Cs.lLOy
unit in the cell. The X-ray pattern of y-Cs-U.O- is listed in appendix CI,
6.6. The neutron analysis of a-Cs-U-Oy
Λ pure sample of a-CsJuyO-, was mounted on the neutron powder diffracto-
meter at the Petten High Flux Reactor. The sample was contained in a
cylindrical vanadium sample holder of 0.2 mm wall thickness and 20 mm
diameter. Monochromatic radiation with a wave length of 2.5715(4) A
was obtained from a copper (111) - plane [4], Soller slits of 10' angular
divergence were mounted between the reactor and the monochromator, and
in front of the BF_ detector, respectively. At room temperature the
neutron profile of the sample was measured frcm 150 to 3850 dmc 2Θ in
steps of 2 dmc (10000 dmc - 360°) in about 4 days. The ratio of the peak
heights to the background was rather low because of the high absorption
of the neutrons by cesium.
The neutron data were used to refine the structure of a-Cs„U20
7
with a program written by Rietveld [5,6]. Applying the profile refinement
method the following quantities were varied: the cell dimensions, the
zero point of the 2Θ scale, the atomic position parameters, an overall
isotropic temperature factor, a scale factor, the half width parameters
- Ρ tan θ + Q tan θ + R where bQ is theθ
P, Q, R from the relation
width at half maximum of a Bragg peak at angle Θ, a parameter which
allows a correction for preferred orientation in the 001 direction and
a parameter which corrects for asymmetric deviations of the observed peak
from a Gaussian shape at low diffraction angles. In the refinement the
scattering lengths 0.85 * 10 cm, 0.558 χ 10 cm and 0.580 χ 10~l2cm
were used for uranium, cesium, and oxygen, respectively [7].
Κ*Λ
• " * ;«*.'.•.%•
. I ' .
-"·-• v « U
—O I
First the structure of a-Cs U_07 was refined in space group C2/m
resulting in an R-factor of 12.9% (R * ΐ w | IQ b s
- I j | / Σ w Io b s
) .
where the profile is converted to integrated intensities Γ63. Since the
neutron intensities are much more sensitive for a change in the oxygen
positions than the X-ray intensities are, the a-Cs-U-O, structure has
also been refined in space group C2 with the neutron data. This refine-
ment did not result in a better R-factor nor in better standard devia-
tions for the least-squares parameters. Therefore C2/m is concluded to
be the better space group for description of the a-Cs_U_O_ structure.
Furthermore the following cell parameters resulted from the C2/m neutron
refinement: a - 14.528(1) X, b - 4.2676(3) X, c - 7.6026(6) X and
β » 112.986(7)°, all in good agreement with the X-ray cell dimensions.
The overall temperature factor was 1.6(1) X and the corrections for
asymmetry of the peaks and preferred orientation were small. The values
of the position parameters are listed in table 6.4 and uranium-oxygen
distances are collected in table 6.2. Figure 6.5 shows the fit between
observed and calculated profile.
Least-squares
Table 6.4.
coordinates of a-CsJ]J) from the neutron data.
Atom
U 4i
Cs 4i
01 4i
02 4i
03 4i
04 4g
x/a
0.1425
0.3935
0.1923
0.4133
0.3055
0.0000
a(x/a)
0.0006
0.0007
0.0007
0.0007
0.0007
-
y/b
0.000
0.000
0.000
0.500
0.000
0.171
a(y/b)
-
-
-
-
-
0.003
z/c
-0.006
0.567
0.252
0.275
0.005
0.000
a(z/c)
0.001
0.001
0.002
0.001
0.002
-
occupancy
1.0
1.0
1.0
1.0
1.0
0.5 j
6.7, Discussion
Especially in the investigation on the cesium diuranate structures a
great influence of the heat treatment, the reaction time, the inclusion
of impurities and the particle size of the materials on the structure of
the sample was found. For example, a second sample of 6-Cs_U_0_ yielded
a unit cell with a - 14.512(2) X, b - 4.2967(3) X, c - 7.535(1) and
• f e s . ••'ν.;'ί.·> ' . ' ^ V
- 1D8D ι
880-j
480 Η
280 4
fCPHB-CS2U207, 300 DEG K. LflMBDfi = 2.571514)
Figure 6.S.
Neutron powder profile of
OBSERVED PROFILE
CSLCULBTED PROFILE
1 0 2° 30 40 SO 60 70 1Ï0 UO
n£Tfl( DEGREES)
to
.-/'• •" . 'ΐ
•?**
-βί-
ο - 113.35° on indexing the X-ray pattern [8], which is significantly
different from the cell constants, reported in section 6.4.
The γ-diuranate formation might also be influenced by the above
mentioned facts, as may be concluded from the description in section 2.3.
Due to the poor crystallinity of γ-CsJJJO7 the unit cell reported in
section 6,5, can be a subcell. In the next chapter it will be seen that
K.U.O- and Rb„U„O_ samples show a splitting of the X-ray lines, causing
these structures to be monoclinxcally deformed. The same might be true
for the y-CsJJ.Oy crystal structure. However, the structure of γ-ϋβ,ϋ,Ο-,
as described by Kovba et al. [1] will not be grossly incorrect: it con-
sists of hexagonal uranium layers and hexagonal cesium layers, linked by
uranyl oxygen atoms. The remaining oxygen atoms in the hexagonal uranium
layer might well be ordered positions, in agreement with the description
for K2
U2°7
a n d Rb2
U2°7
i n t h e s e c t i o n s 7.3.3 and 7.5.
When tables 6.1 and 6.4 are compared, the X-ray refinement and the
neutron refinement of the a-Cs2U.O
7 structure result in the same coor-
dinates. From the neutron data the oxygen coordinates are obtained more
precisely as could be expected from the neutron scattering lengths.
Both refinements result in a good agreement of the U-0 distances, which
are listed in table 6.2.
From the crystal structures presented in this paper the phase
transformation from the α-diuranate into the β-diuranate can be understood
as follows: the 0,-atom, which is statistically disordered in the
α-structure (figure 6.3.a) occupies a fixed position in the fj-structure
(figure 6.3.b). This results in a small rearrangement of all atoms in
the diuranate structure modifying also the cell parameters.
The most surprising result of this study is the arrangement of the
uranium atoms in the diuranate structures. The other cesium polyuranates
contain pseudo-hexagonal uranium layers [chapter IV,V] and the mono-
uranate contains tetragonal uranium layers [chapter V]. The roonoclinic
diuranate crystal structures are built up from layers which contain
hexagonal as well as tetragonal arrangements of uranium atoms. This kind
of uranium arrangement in layers has never been reported before.
References
[1] L.H. Kovba, I.A. Murav'eva and A.S. Orlova, Radiokhimiya, 16,
648 (1974).
.- ί
τ >'
ι.':,
-64-
[2] D.T. Cromer and J.T. Waber, Acta Cryst., J£, 104 (1965).
[3] D.T. Cromer, Acta Cryst., JJ3, 17 (1965).
CA] B.O. Loopstra, Nucl. Instrum. Methods, 44^ 181 (1966).
[53 H.M. Rietveld, Acta Cryst., n, 151 (1967).
[6] H.M. Rietveld, Α Program for the Refinement of Nuclear and Magnetic
Structures by the Profile Method, update 1972 (1969).
[7] G.E. Bacon, Acta Cryst., A28, 357 (1972).
[8] E.H.P. Cordfunke, A.B. van Egtnond and G. van Voorst,
J. inorg. nucl. Chem., 37, 1433 (1975).
I..-!:'45
'ji.
Γ
9 . · '
V ^ : * * •
Jtivi'-IHX&R:-. i'éL'^jsüi;.'ο,
-65- I. :
CHAPTER VII. POTASSIUM AND RUBIDIUM URANATES
7.1. Introduction
In the foregoing chapters the crystal structures of the cesium uranateu
have been described. To examine the influence of the alkali-metal ion
radius on the composition and the crystal structures of the uranates
structural information on the lithium, sodium, potassium and rubidium
uranates is needed. Recently systematic investigations on lithium and
sodium uranates have been published [1,2], A literature study of the
potassium and rubidium uranates yielded several incomplete and even con-
flicting data. Therefore it was decided to investigate the K-U-0 and
Rb-U-0 systems systematically. The results of this study are presented
in this chapter.
i
7.2. Experimental
In general the experimental methods described in chapter II for the
cesium uranates were used in this study.
Carefully ground mixtures of amorphous U0_ and alkali carbonate reacted
completely at 700 C within about 50 hours. The metal/uranium atomic
ratios were accurately fixed in the starting mixtures as 4.0, 2.0, 1.5
and from 1.0 down to 0.1 in steps of 0.1. All samples were handled in a
dry box, although only the M.UO, samples are rather hygroscopic. The
uranium content was analysed in some cases according to the procedure
described by Eberle et al. [7],
7.3. Hexavalent uranates
7.3.Κ The potassium uranate system
From Guinier films of the reacted mixtures it was concluded that four
distinct phases at K/U ratios 2.0, 1.0, 0.5 and 0.286 exist in air up
to at least 700°C. Since uranium is entirely in the hexavalent state,
the formulae of these compounds are K-UO,, KpU„O_, Κο^Δ^ιι an(
* ^9^7^99
*)This chapter is a revised version of the paper by A.B. van Egmond
and E.H.P. Cordfunke, J, inorg. nucl. Chem., in the press.
e»?»«Bfflisaaw)Rs«5»a<?;if'.-';i'i·:
7.1.
FORMULA
K2UO4
K 2 U 2 O 7
Wl3
K2Ü7°22
Rb2UO4
Rb2U2O7
RWl3
RW>22
Crystal data of potassium
AXES j a.b.c d) + )
ANGLES: α,β,γ (degrees)
4.3319(3) 4.3319(3) 13.185(1)90 90 90
6.9252(5) 7.9729(9) 6.9920(7)90 109.623(8) 90
14.307(1) 14.307(1) 13.998(2)90 90 120
6.9500(6) 19.525(2) 7.2121(6)90 90 90
4.3548(3) 4.3548(3) 13.869(2)90 90 90
6.9472(5) 8.0175(6) 7.3276(8)90 108.635(8) 90
14.339(1) 14.339(1) 14.311(2)90 90 120
6.9586(4) 19.609(1) 7.2781(5)90 90 90
and rubidium wcanates(VI) **'
CRYSTALCLASS
TETRAG.
MONOCL.
HEXAG.
0.RHOMB.
TETRAG.
MONOCL.
HEXAG.
0.RHOMB.
SPACE * * * >GROUP
1 4 /nnnm
P2,/m
P63/m
Pbam
I4/mmm
P2,/m
P63/m
Pbam
ζ
2
2
8
2
2
2
8
2
d calc
5.103(1)
6.085(1)
6.629(1)
7.114(1)
5.972(1)
6.519(1)
6.939(1)
7.320(1)
Μ * }
96
44
27
45
99
29
30
59
*)
**)
***)
t)
See ref. [33.
The complete X-ray patterns are published in the appendices.
Space group with highest symmetry is indicated [4],
Unit cell parameters have been calibrated with α-quartz.
A single crystal of α-quartz showed on a PW1100 single crystal diffractometer
a hexagonal unit cell with a * 4.913(2) 8 and c « 5.405(1) 8.
liüüi'·'^
-67-
respectively; their colours vary from yellow to orange. The X-ray
patterns could be indexed from Guinier film data of samples which were
heated at 700°C for at least 100 hours. The unit-cell parameters, which
were calibrated with α-quartz, and the calculated densities of the
potassium uranates(VI) are listed in table 7.1. The X-ray pattern of a
mixture with K/U ratio 4.0 showed some unidentified weak lines at the
initial stages of the calcination, but these lines disappeared after
continued heating, indicating that a compound K.UO, is not stable at
700°C in air. The X-ray spectra of the potassium uranates are given in
the appendices C12 - C16.
·.· -1
This system is very similar to the potassium uranate system. Thus, the
uranates(VI) RbJDO,, Rb„U_O7, Rb-U.O.. and Rb
2U
7O„
2 have been found to
exist, their colours varying from yellow to orange. The formation of the
rubidium uranates takes also place in air at 700°C. The results of the
indexing of the X-ray powder patterns are given in table 7.1. It is
evident that all rubidium uranates are isostructural with the corre-
sponding potassium uranates. The X-ray patterns of the rubidium
uranates are presented in the appendices C7 - CI 1.
7.3.3. The crystal structures of M_U,0 and M-U-O (M=K,Rb)
The crystal structure of K-U.O.» has been investigated by powder dif-
fractometry. According to the systematic extinctions the space group can
be P6_/m, P6, or P6.22 [4]. From a Patterson synthesis, based on 29 re-
flections, a layer structure with sheets perpendicular to the c-axis and
consisting of 13 uranyl groups could be found. This model excludes the
P6.22 space group. The remaining uranium atoms and the potassium atoms
could not be located. Structure models with different interlayer posi-
tions for uranium and potassium never resulted in converging refinements.
In all least-squares calculations, using scattering factors published by
Cromer and Waber [5,6], a rather large shift in one of the uranium atoms
was found, as indicated in figure 7.1. Assuming the interlayer uranium
atoms to be grouped in uranyl groups the hexagonal layer composition can
be deduced to be (UO-^QO-Q, as already suggested by Kovba and Trunova
[28].
11
ι-, > j
IS ' 'Λ '.
Κ. ' ;/•$
V. i
-68-
Figure 7.1.
Uranium positions in pseudo hexagonal (UO )-J)^^ layers in
ίίηί/.Ο . Ideal hexagonal positions are given by the lattice
(a = b = 14.Z07U) &, space group P6_/m; the following
positions have been plotted: (.OO,.OO3.OO)
} (.32,.24,.02)
and (.60,.16,.01)).
The unit cells of potassium and rubidium diuranate have been reported
to be rhombohedral [13,15,17,21,24], From our results it follovs that
both diuranates crystallize in the monoclinic crystal system with space
group P2./m or P2 . The strong subcell reflections (hkl : k= 2n) lead to
the following positions for the metal atoms in P2./m: uranium in 2a and
2e (x = 0.50, z = 0.00), potassium (or rubidium) in 2d and 2e (x = 0.00,
z=0.50). In agreement with the sodium diuranate structure C23] the
oxygen atoms in the layers may be located in 2b, 2e (x = 0.21, z = 0.00)
and 2e (x = 0.79, z = 0.00) and the uranyl oxygen atoms in 4f (x = 0.10,
= 0.00, z = 0.28) and 4f (x = 0.60, y = 0.25, 0.28)
I- - ih
I
t
'-'1•3
51
: :·.ν -1
M i •-
H-.i:·?. :.
-69 -
7.4. Pentavalent uranates
The formation of the pentavalent KUO, and RbUO, by the reaction of U02
with equimolar amounts of K?UO, and RbjUO, respectively at about 800 C
in inert atmosphere has already been described by RÜdorff et al. [9,10]. •
The authors mention that they could not prepare CsUO, in this way. This
is confirmed in our experiments, the compound Cs2U,O12 being formed in-
stead [chapter II,III]. On the other hand, heating Rb„U,0 „ or mixtures
of Rb2UO, and U,0„ in the molar ratio Rb/U = 2.0 in inert atmosphere
does not yield RbJl.O.,» but a mixture of RbUO, and UO,,. The formation
takes place via several intermediate metastable phases.
Although KUO, and RbUO, can also be obtained by thermal decomposition
of K2U2O or Rb2U2O7 at 850°-900°C, the reaction is very slow, U02 being
formed gradually by the simultaneous decomposition of KUO, or RbUO, into
UO2< Decomposition into U02 takes place rapidly above 1050 C. Heating of
the uranates(VI) in hydrogen yields U02 at already low temperatures
(~ 600°C).
The X-ray patterns of both KUO, and RbUO, can be indexed with a
perovskite-type cell, space group P23, in agreement with the observations
by RÜdorff et al. [9,10], The data are summarized in table 7.2.
Table 7.2.
Crystal data of KUOV ando
compound
KUO3
RbUO.
colour
greybrown
redbrown
cubic cell coi
RÜdorff [9,10]
4.290
4.323
istant (X)
this study
4.2966(3)
4.3275(3)
ζ
1
1
X-ray
density
6.806(1)
7.635(1)
7.5. Discussion
In the literature many investigations on the potassium and rubidium
uranates are found. The following potassium uranates(VI) are reported to
exist: K2Ug0
2 5 [19], K ^ O ^ [18,27], K^Ojg [13,16,21,26],
K2
U4°13 CI3,I6,26.273, K
2
U3°io C 11,16,19,24], K ^ ^ [ 11-19,2», 23-273,
K2UO
4 [14,16,17,19] and K^UOg [12,16,20].
However, in this investigation no indication was found for a compound Sift'1
'J-•11
-70-
mm
KnU_O-,. or K„0. nUO- (n>8) as mentioned by Anderson [19]. Kovba [27]
reported by unit cell parameters of K
2
U7^22*
an(* t*
ie ^
u" · ^
=rystal struc-
ture of this compound some years later [183. Kovba's results are con-
firmed by ours, K,.U_0„2 being isostructural with Cs.U 0.. [chaptsr V],
The phase region K„O. η UO„ (3 Sη i 6) has been a source of much
confusion. Kovba and Churbakova [21] proposed a solid solution with limits
K2O(UO
3) and K,U,0.-, called "tri-hexauranate". Recently the existence
of this solid solution has been denied by Kovba [27], However, Toussaint
and Avogadro [24] yet reported a triuranate in agreement with Kovba's
earlier results [21]. Efremova et al. [16] reported the compounds K„U,0 ,
K„ü,0 and Κ2
υ3
Οΐ0
b u t A n d e r s o n E
1 9^ interpreted the infra-red spectrum
of K.U.O _ as a superposition of the spectra of Κ2
ϋ3°10
a n d a n* 8
h e r
uranate K.l) 0 ,. Furthermore Anderson [19] announced a single-crystal
analysis of Ko^Vin w^^
c n» however, was not completed [32]. Allpress et
al. [13] described the decomposition of potassium halogeno-uranates into
K2
U6°19
a n d K2
U4°13'
b u t L u c a s ^ "
d e t e c t e d K2
U3°l0
i n s i m^
l a r decompo-
sition experiments. Kepert [26] also described L O O . and K_U,0._ to be
formed in the potassium uranate system.
Our results are in contradiction with most of these statements:
besides K„U,0 the only stable potassium polyuranate found is K_U,O._.
The indexed unit cell is in agreement with the results of Kovba [27],
Kovba and Trunova [28] suggested (U 0
2^n°20
l a v e r s t o b e present in the
Rb.U.O _ crystal structure, which is in agreement with our results for
Wl3·
As already shown' KJ3J)- crystallizes in the monoclinic crystal
system. Anderson [19] already suggested that the previously assumed
rhombohedral cell must be a subcell, which was confirmed by Hoekstra [14].
The subcell indicates that the metal atoms form pseudo-hexagonal layers,
but the oxygen atoms in the U-0 layers are no longer forced to be dis-
tributed in fractionally occupied positions in an eightfold uranium
coordination, as was assumed by Kovba [23,27]. No indication for a phase
transformation in the diuranate structure [8] has been found.
The compound K2^4 ^
s well-known in the literature. It has a tetra-
gonal uranium layer structure and belongs to the K.NiF, class [8,30].
From our high-temperature X-ray work no indication has been found for a
phase transition of K-UO, into a cubic form as described by Kovba et
al. [20]. Probably these authors observed the decomposition of K„U0,
into KÜ0-, as may follow from their reported cell constant.
i:'i;
PW?;·
I
= " :f
'Λ. , e i '-.--- · if-
:
'ίΤ
f ,'1
'r'
JL' J .
' ''\
*«·#··!--.-Ui,
-71-
The existence of K.UO , as reported by Efremova et al. [12,16,20],
could not be confirmed, which is in agreement with results of Kepert [26]
and Hoekstra and Siegel [25],
As in the case of the potassium uranates several rubidium uranates
have been mentioned in the literature. Our results agree with a recent
publication of Kovba and Trunova [28], except for the unit cell of
rubidium diuranate. A compound Rb.U.O.-, described as the final product
of the decomposition of Rb.UO-Cl, [29] could not be prepared by us.
Probably the final product of these decomposition reactions is Rb„U,0 „,
as indicated by Allpress et al. [I3J.
Finally, the raonoclinic indexing of potassium and rubidium
diuranate, given in this paper, is close to the indexing of Kovba [23]
and Cordfunke and Loopstra [2] for the sodium diuranate, However on the
basis of the primitive monoclinic unit cell for potassium and rubidium
diuranate, it is still not possible to index all the reflections given
for Na.U.O- in the latter paper.
References
[1] J. Hauck, J. inorg. nucl. Chem., _36. 2291 (1974).
[2] E.H.F. Cordfunke and B.O. Loopstra, J. inorg. nucl. Chem., 33,
2427 (1971).
[3] P.M. de Wolff, J. Appl. Cryst., ]_, 108 (1968).
[4] International Tables for X-ray Crystallography, Birmingham (1962).
[5] D.T. Cromer and J.T, Waber, Acta Cryst., Jj5, 104 (1965).
[6] D.T. Cromer, Acta Cryst., JJJ, 17 (1965).
[7] A.R. Eberle, M.W. Lerner, C.G. Goldbeck and C.J. Rodden,
NBL-report no. 252, (1970).
[8] Investigations in the field of Uranium Chemistry, V.I. Spitsyn,
editor, ANL-report Trans-33 (1961).
[9] W. Rüdorff, S. Kemmler-Sack and H. Leutner, Angew. Chem., 74,
429 (1962).
[10] S. Kemmler-Sack and W. Rüdorff, Ζ. anorg. allg. Chem., 354,
255 (1967).
[11] J. Lucas, Rev. Chim. miner., I, 479 (1964).
ft' -?·:
• • β
- ' 'ή
•"· '·>ν-.'*-
-72-
[12] K.M. Efremova, E.Α. Ippolitova and Yu.P. Simanov, Vestnik Moskovsk.
Univ., Khim. Set., 24, 57 (1969).
[13] J.G. Allpress, J.S. Anderson and A.N. Hambly, J. inorg. nucl. Chem.,
30, 1195 (1968).
[14] H.R. Hoekstra., J. inorg. nucl. Chem., 7, 801 (1965).
[15] N.C. Jayadevan, K.D. Singh Mudher and D.M. Chackraburthy, BARC-
report-726, India, Bombay (1974).
[16] K.M. Efremova, E.A, Ippolitova, Yu.P. Simanov and V.I. Spitsyn,
Dokl. Akad. Nauk. SSSR, j24_, 1057 (1959).
[17] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn,
Dokl. Akad. Nauk. SSSR, J_20, 1042 (1958).
[18] L.M. Kovba, Eh. Strukt. Khim., _Π, 256 (1972).
[19] J.S. Anderson, Chimia, £3, 438 (1969).
[20] L.M. Kovba, E.A. Ippolitova and Yu.P· Simanov, Zh. Strukt. Khim.,
2, 211 (1961).
[21] L.M. Kovba and T.I. Churbakova, Zh. Strukt. Khim., 2, 585 (1961).
[22] L.M. Kovba, Radiokhimiya, J_3, 309 (1971).
[23] L.M. Kovba, Radiokhimiya, JU, 727 (1972).
[24] C.J. Toussaint and A. Avogadro, J. inorg. nucl. Chem., 3£, 781 (1974).
[25] H.R. Hoekstra and S. Siegel, J. inorg. nucl. Chem., 6, 691 (1964).
[26] D.G. Kepert, Thesis, University of Melbourne (I960); cited in [25].
[27] L.M. Kovba, Radiokhimiya, J_2, 522 (1970).
[28] L.M. Kovba and V.I. Trunova,' Radiokhiraiya, J^, 773 (1971).
[29] M.P. Vorobei, A.S. Bevz and O.V. Skiba, Zh. Fizich. Khim., 4£, 2434
(1974).
[30] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn,
Russ. Jrnl. Phys. Chem., 35_, 275 (1961).
[31] W.H. Zachariasen, MDDL-report, 1152 (1946).
[32] J.S. Anderson, private communication (1975),
τ.-
V'f
1
'.•Λ 11
-73-
CHAPTER VIII. CRYSTAL CHEMISTRY OF THE ALKALI URANATES
8.1. Introduction
In the foregoing chapters of this thesis the crystal structures of most
of the cesium uranates, which were introduced in chapter II, have been
described. In addition, some attention has been paid to some regions of
the Cs-U-0 phase diagram, for instance the phase transition a + B - C s - U ^
and the phase region Cs-U.O.- - Cs-U,-0]/:· F°* convenience the most
relevant structural information of the unit cells of the cesium uranates
is listed in table 8.1. It also contains information on two compounds,
which were given the formulae Cs„U.0.o and Cs_U„O„., in chapter II. These
z o is ί y ii
compounds have not been investigated in detail because the influence of
the (partial) oxygen pressure on their formation is not known accurately.
Rubidium and potassium uranates have been discussed in chapter VII.
The Na-U-0 system has been investigated systematically by Cordfunke
and Loopstra \T\. Compounds with formulae Na.UO , Na.UO, (a and β),Na„U„C· (α,β and γ) and Na,U_O
ι ζ. ι b /were found to exist as distinct sodium
uranates(VI). Upon reduction of the hexavalent sodium uranates two
uranates(V) with formulae NaUO, and Na_UO, are formed.
The lithium-uranate system is more complicated than had been assumed
for a long time [3]. Recently, Hauck Γ4] has reinvestigated the Li-U-0
system and surveyed the literature on lithium uranates. In the phase
diagram of the pseudo-binary system Li.O-UO, the following uranates can
be distinguished: Li,UO, (a and 0), Li.UO., Li„UO., Li,UrO,_ (α,β and v),
ob H j i. t\ ο j Ιο
Li„U_O _ and Li?U,O.q. Upon reduction of the lithium uranates(Vl),
according to Kemml^r-Sack [51, a number of stable compounds is found:
Li7UO,, Li.UOj, LiUO.j, Li„U,0 . and a compound with composition
0.15 .
In figure 8.1 the various compositions of the hexavalent alkali
uranates are summarized. In this chapter the alkali uranates will be
discussed from a crystallographic point of view.
8.2. Structural characteristics of uranates(VI)
In 1960 Kovba Γ6] published a study entitled: "the regularities in the
structures of uranates and their relation to the properties of the
uranates". He proposed three characteristic types of uranate motifs,
ι?
, ' • ';· • '
mmn iiiiiwiTW»r
Table 8.1. Crystal data of ceeiwn uranates.
Formula
Cs2U04
a-Cs2U2O7
B-Cs2U2O7*)
Ï-CS2U2O7
CS4U5°17
C-2°*°I3">
Cs2U7°22
CS2U,5°46
a-Cs2U40!2
6-Cs2U4°.2*)
Y-CS2U4O12*>
CS2U6°18
CS2U,O27
CrystalClass
tetragonal
raonoclinic
monoclinic
hexagonal
orthorhombic
orthorliombic
monoclinic
orthorhombic
orthorhombic
rhombohedral
monoclinic
cubic
monoclinic
orthorhombic
Spacegroup
14 Aram
C2/m
C2An
P6.,/mcc
Pbcn
Cracm
C2/m
Pbam
Cm» a
R3m
P2,
Fd3m
primitive
Axes : a.b.c &) + )
Angles: α,Β,γ (degrees)
4.3917(6) 4.3917(6) 14.804(3)
90 90 90
14.528(1) 4.2676(3) 7.6026(6)
90 112.986(7) 90
14.516(2) 4.3199(6) 7.465(1)
90 113.78(1) 90
4.108(1) 4.108(1) 14.646(5)
90 90 120
18.776(4) 7.070(1) 14.958(3)
90 90 90
13.494(2) 15.476(2) 7.911(2)
90 90 90
13.465(2) 15.561(2) 7.964(2)
90 92.78(1) 90
6.949(1) 19.711(2) 7.3955(8)
90 90 90
14.686(3) 13.422(2) 19.752(3)
90 90 90
10.9623(6) 10.9623(6) 10.9623(6)
89.402(7) 89.402(7) 89.402(7)
7.886(1) 8.002(1) 10.793(2)
90 92.62(1) 90
11.2295(6) 11.2295(6) 11.2295(6)
90 90 90
4.137(1) 13.471(1) 8.089(1)
90 90.37(1) 90
14.956(4) 10.571CJ) 3.9856(7)
90 90 90
"20see Γ1 ]
39
50
51
-
25
33
30
37
23
44
38
88
60
16
ζ
2
2
2
1
4
4.8
4
2
4
2
4
1
I
calc
6.604
6.535
6.619
6.624
6.669
6.879
6.822
7.488
7.800
7.110
6.882
6.613
7.300
7.484
d2 0
meas
-
-
6.52(12)
-
6.62(15)
6.82(17)
-
-
7.16(20)
-
-
-
*) unit cells of 3-Cs2U
2O
? at 600°C, g-Cs
2U
4O
l2 at 660°C, y - C s ^ O ^ at 880 C.
} For Cs
oU.0,
o and Cs„U
cO,, subcells are given; CsJJ.O., at 800°C, Cs/U-ratio was 0.42.
.» 2 4 13 2 5 16 Ζ 5 ID
r
t)Unit cell parameters have been calibrated with α-quartz, see note at table 7.1.
MMa^
8.0 4.0 2.0 1.0 05 025 0.125M/U-ratio
Figure 8.1.
The compositions of alkali uranates(VI).
all with composition [(U02)0_]:
1. hexagonal uranium layers,
2. tetragonal uranium layers,
3. infinite uranium chains.
Especially in the alkaline-earth uranates(VI) Kovba pointed out a
systematic influence of the metal-ion radius on the crystal structure of
MUO, compounds. However, in case of the alkali uranates such a regularity
was not obtained.
Some years later Keller [7] gave a systematic description of crystal
structures and bonding in uranates(VI) in his Habilitationsschrift "Uber
die Festkörperchemie der Actiniden-Oxide". After an extensive literature
survey Keller distinguished six types of uranium-oxygen motifs, also
present in other metal-actinide-oxygen compounds:
fatfe
-76-
(a)
Λ. s
a . •
Figure 8.2.
Uranium-oxygen motifs according to Keller [7]j see section 8.2:(pseudo)-hexagonal layers of (UOgJOg-cubes with composition i(UO2)O2l (a),(pseudo)-tetragonal layers of (W^^O^-octahedra with composition[(UO2)C>2~} (b), endless chains of (U02)0^-oatahedra with composition[Ci/OgJOg] (°)>
and endless chains of (UO4)02-octahedra with composition
L(UO4)O1 (d). The black spheres are more or less in one plane; the largeones designate oxygen atoms, the small ones uranium atoms. The open spheresdesignate oxygen atoms above and below the plane of the black spheres^forming the uranyl groups in (a), (b) and (c), and forming part of the"Og-square" in (d).
ιΐ'ρι-'·:·'•:[;&/•:. *'..:„'y' ."ί"'ν.'·.'.ι.;.,.
:.
''•Λ' - .-V--..> iafeij^
-77-
1. (pseudo)-hexagonal layers of (UO_)O,-cubes with composition [(UO.JO^]
2. (pseudo)-tetragonal layers of (UO,)0,-octahedra with composition
[(UO2)O
2];
3, endless chains of (UO-)O.-octahedra with composition [1(02)02];
4. endless chains of (UO,)0.-octahedra with composition [(UO.)O];
5. isolated UO,-octahedra, linked by metal ions;o
,6+6. regular UO,-polyhedra with a statistical distribution of U and
t>
metal ions.
It should be noted that the last type referred to a cubic compound
a-Na,UO_, which, however, does not exist, as was shown by Cordfunke and
Loopstra Γ2]. Also the other actinide compounds a-Na,XO5 (X*Np.Pu.Am),
which were mentioned by Keller, have not been published in the literature.
The first four types are shown in figure 8.2.
Keller also extended the group of alkaline-earth uranates(VI),
described by Kovba [6], to a group of bivalent-metal uranates(VI). In
this way he found that the uranium-oxygen bonding type in these compounds
is determined by the radius of the metal ion, as is shown in table 8.2,
•A
• I
Table 8.2.
The influence of the cation radius on the uranium-oxygen
bonding type, according to Keller i7l. The type numbers are
explained in section 8.2.
uranate(VI)
L i2
U O4
N a2
U 04
K2
U O4
Rb2UO
4
Cs2UO
4
cationradius (A)
0.68
0.98
1.33
1.49
1.65
type
1+2
2+3
2
2
2
uranate(VI)
MgU04
CdUO^
CaU04
SrU04
PbU04
BaUO4
cation
radius (A)
0.74
0.99
1.04
1.20
1.26
1.38
type
3
3+1
1
1+2
2
2
Both, Kovba and Keller, emphasized in their studies that some minimum
uranium-uranium distance corresponds to each type of uraniutn-oxygen
bonding.
'-.' w• * * ί κ •„Siari.'-isi
V J L L ^ •'•'·*
-78-
Independently from Keller Prigent and Lucas [8] distinguished
three types of crystal structures for alkali mono- and diuranates,
similar to those proposed by Kovba Γ63.
8.3. Uranium(-oxygen) motifs in alkali uranates(VI)
The alkali-uranate(VI) crystal structures have been restudied since a
decade and have been established fairly well at present. Only the lithium
uranate structures have not yet been elucidated entirely Γ A]. Neverthe-
less it is possible by now to characterize the alkali-uranate(VI) crystal
structures along similar lines as was done by Keller in the case of the
alkaline-earth uranates. This characterization of the crystal structures
is given in figure 8.3.
• ït-':'\: J I.* '
1.0M/U-ratk)
0.5 025 0.125
Figure 8.3.
Struatuval characteristics of alkali uranates(VI);
(for explanation of symbols, see section 8.3).
The following types have been assumed to be characteristic:
1. L, stands for a (pseudo)-hexagonal arrangement of the uranium atoms
in mutually isolated layers;
2. L means a (pseudo)-tetragonal arrangement of the uranium atoms in
mutually isolated layers;
•u···"
J
. '· •/
•il %
-79-
3. CJJQ o stands for mutually isolated endless chains of (U0_)0,-
octahedra, sharing oxygen edges, with composition [(U0_)0_]; the
chains are linked by metal ions;
4. CjjQ o i-s short for mutually isolated endless chains of (UO^i^-
octahedra, sharing corners, with composition C(UO,)O]; the chains are
linked by metal ions;
5. ISOL means a structure, which contains mutually isolated
UO,-octahedra, linked by metal ions;
6. 3D stands for a structure type, which can be regarded as an ot-U_Og
three-dimensional network of uranium-oxygen bonds, in which alkali-
metal ions have been introduced.
In the first two types no mention is made of the oxygen atoms in
the (pseudo)-hexagonal or (pseudo)-tetragonal layers, which makes these
two types not entirely comparable to the corresponding types of Keller,
that were mentioned in section 8.2. Keller used the CaUO, crystal struc-
ture as an example of the hexagonal layers, in which the uranium atoms
are coordinated by eight oxygen atoms, forming (IK^O^-cubes. However,
this coordination is not found in any of the (pseudo)-hexagonal layers
of the alkali-uranate(VI) structures. In these structures the uranium
atoms are coordinated by six or seven oxygen atoms.
The types 3, 4 and 5 introduced here are comparable to the cor-
responding Keller types, but type 3D is not the same as type 6 of the
Keller classification, which is not encountered in any real crystal
structure, as was mentioned in section 8.2.
In figure 8.3 several of these type symbols have been introduced
for one compound, when that compound exhibits two or more phases
(CsJ 0?, Na-U.O-, Na.UO,, Li-UO,) with essentially different crystal
structures.
From figure 8.3 it is evident that the crystal structures of the
alkali uranates(VI) can be classified by the following rules:
1. with increasing alkali-metal radius a layer structure is preferred;
for large alkali-metal ions chain structures or isolated-octahedra
structures are not found;
2. in layer structures a hexagonal arrangement of the uranium atoms is
found for M/U < 2.0, while tetragonal layers occur for a ratio
M/U » 2.0; only in the case of cesium a mixed tetragonal-hexagonal
layer structure is found to be the stable phase (in figure 8.3 the
symbol L is used);
3. small alkali ions can be introduced in an ct-U_OQ lattice.J O
, J*, \- • ;·
' -".•''» •.! ·· 1 -
-80-
The first rule can be understood from the fact that small ions
prefer low coordination numbers, whereas large ions prefer high coordi-
nation numbers.
On comparison of the figures 8.2a and 8.2b rule 2 is more or less
self-evident: the area covered in a tetragonal uranium motif is larger
than the area covered by a hexagonal uranium motif, given a fixed number
of uranium atoms. Consequently the tetragonal layers leave more space for
penetration of the alkali-metal ions into the layer.
The third rule follows from the neutron study of the ot-UO, crystal
structure by Greaves and Fender [9]. These authors established the a-U0_
structure to be a uranium-deficient a-U-0o structure.
These rules will be illustrated by a detailed discussion of
figure 8.3 in the following sections.
Μ
%
In this region of figure 8.3 two almost complete series with formulae
Μ UO, and M_U 0 are present.
Starting with the hexagonal layer structures of Na2U„Ü7 [10] and of
Κ U 0 and Rb.U 0? [chapter VII], in which the interlayers have been
completely filled with alkali ions, it is plausible that, on increasing
the M/U-ratio and preserving a layer structure, the uranium layers have
to form tetragonal motifs.
In the case of cesium a metastable hexagonal phase Y-CS_U.O7 is
found. Due to the large radius of the cesium ion the average U-U dis-
tance in the hexagonal uranium layers has been increased to about 4.1 X
(in Na2U
2O
?, K^O.,, R b ^ O ^ 3.95 - 4.00 R). This causes the introduc-
tion of tetragonal motifs in the stable phases of Cs U_0_, resulting in
layers with both hexagonal and tetragonal motifs. A stable (pseudo)-
hexagonal structure is also possible when cesium is removed from the
interlayer, i.e. the Cs/U ratio is lowered. This explains the existence
of the (pseudo)-hexagonal crystal structure of Cs.OJ) .
Since β-Na.UO, is a metastable phase [2], the formation of
tetragonal layers in Li_UO, seems doubtful. However, there is evidence
that the lithium ions are in the tetrahedral interstices rather than in
the ninefold coordination of the K_NiF,-structure [11]. Both β-Na.UO,
and Li Ü0, are orthorhombically deformed, which is due to the tendency
for low coordination numbers for sodium and lithium ions. This tendency
I
m
-81-
also makes plausible that the chain structure of a-Na.UO, [12] is stable
with respect to β-Na.UO, [2].
Whereas in the case of cesium the Cs/U-ratio is lowered from 1.0 to
0.8 to form the (pseudo)-hexagonal structure of Cs.U.O 7 > the lithium
ion is so small that additional lithium oxide can be introduced in the
hypothetical lithium-diuranate structure, increasing the Li/U-ratio to
1.2 in the compound Li,UcO
1Q (= LiU-
Q-0,) [4],
On the other hand one could assume uranium to be introduced in the
interlayer of the sodium diuranate structure, to form Na.U 0,,, but
evidence for this suggestion is not found in the literature [2].
\''•).•?'
Since lithium and sodium prefer tetrahedral coordination by oxygen the
chain structures of Li.UO- and Na.UO can be understood. Potassium,
rubidium and cesium are too large for tetrahedral coordination, explaining
why the meso-uranates(VI) of these elements have never been found (see
chapter VII, [13-15], chapter II).
In Li,UO, (a and g) so many lithium ions are present in the struc-
ture, that the chains are broken up and isolated UO.-octahedra are found
o
in the structure [16]. The arrangement of the UO,-octahiidra in Li,U0, is
different from the arrangement in Ba_U0,, which was used by Keller [7]
as an example of an isolated ϋθ,-octahedra structure (the crystal struc-
ture of Ba,UO, might be slightly more complex than was assumed by
Keller [17]).
The small lithium ion can be introduced in the ci-U„0o structure without
disturbing the three-dimensional uranium-oxygen lattice [4]. The crystal
structure of a-U.0o is shown schematically in figure 8.4a.
3 is
For sodium the same structural fact was established by Greaves
et al. [18], who prepared a sodium uranate(VI) with the formula
Na u|_v/6°3» °* x 0.165. By electron diffraction they found the sodium
ions to occupy the uranium vacancies in the a-UO. structure [9,18]. Β
Because of the variable Na/U-ratio this compound has not been included
in figure 8.3.
In the LiJJfp.n structure [3,4] the lithium ions are probably
statistically distributed, whereas Li-U.O.» can be regarded as a three- teloft
" 7
'..•;-^fe;;„ .'^.ΐ.,'.-τ'.-^^ίΐ'^νν^ * ; i / ; . - •'·'
I « * J
: ' " * v •'• •'•',••." · ' " l' 4 • •••·•'
r^..;S£'':'--
:.""
v''
!K" ·; '•" " ^"'/Ail> k*^^taa^ii-iix£'i
f, ,.«.,;<
•• " K i•• ν '
ü:'.'. •'S'I. • 'S
•••».
•m ι
- 8 2 -
• (a) · (b)
Figure 8. 4.
•Schematic view on the orientation of the uranyl groups in the crystal
structures of a-U„Co (a) and'• &-U0- (b). In a-U-O. 120-221 a three-o O o o o
dimensional uranium-oxygen network is obtained by -O-U-0-U-O-chains
connecting the hexagonal uranium layers; in B-UO [231 the layero
character is preserved by the orientation of the uranyl groups in the
interlayer.
i :
dimensional uranium-oxygen network, in which lithium is introduced in
an ordered way [19]. However, little is known with certainty about the
lithium positions in these structures, because lithium cannot be located
by X-ray powder diffraction and neutron diffraction studies concerning
these compounds have not been performed.
.3. A. and the series (M = K,Rb,Cs)
Cesium can also be introduced in a uranium-oxide structure in a low con-
centration (Cs/U = 0.133), but the cesium ion is too large to fit in the
a-U30„ lattice (figure 8.4a). Therefore the host structure for Cs.U
50 ,
is the β-uo., structure, which is built up from (pseudo)-hexagonal
uranium layers, connected by uranyl groups, as is shown in figure 8.4b.
The M.U7O
2 2 structures are related in a similar way to the β-UO,
structure. The interlayers of these uranate structures contain uranyl
groups and alkali ions in equal amounts (see chapter V, VII, [24]).
When the crystal structures of Cs.U.O _ and Cs' U_0„. are compared,
a "simple" structural relation is found: half of the cesium positions in
Cs.U.O _ have been occupied by uranyl groups in Cs.UyO-.. This relation
is shown in figure 8.5. Applying this procedure once more to the Cs_U70
i >
-•'•'V : ' 'ét .··'•-'t'
I-83-
• fi• • ' /
Figure 8.5.
The interlayers in Cs^ü^O^ (a) and Cs8U?Og2 (b). The Cs^^O^ inter-
layer is formed by oesi:m atoms only; the interlayev in the Cs U?0^
structure consists of cesium atoms and uranyl groups (compare to
figures b.P, and 6.3). Cs;:,U„(),,„ has been assumed to be isostruotuval
with κφ?0νΑ \'λ4Λ.
structure, transformed to the large unit cell of the cesium uranate
with the lowest Cs/U-ratio, a formula "Cs„U,fi0 " can be deduced [25].
From the fact that the formula of this compound is found to be CS-U..O.
2 + 2 15 4
[chapter V], one has to assume that a uranyl group (UO„) occupies a
slightly larger area in the interlayer than a cesium atom.
•'•'•• é'
•'"•&,
§i2i5i_The_tetra-uranate_region
In K-U.O.™ and Rb„ü,O._ the interlayer is occupied by alkali ions and
uranyl groups in the ratio 8/3 (chapter VII, [26.]). Therefore the distance
between the uranium layers is mainly determined by the radius of the
-Jfc. *
i»'.Tig»grr<frnTt7Wg^'i»W:'^^^'T-'*¥T,"*"SgllPi>W>l'' '-TT 'r7fCli' n.", :
..... t."
I-84-
alkali ion. Apparently when the potassium or rubidium atoms are replaced
by cesium atoms, the distance between the uranium layers becomes too
large for interlayer uranyl groups to form a stable coordination with
the oxygen atoms of the uranyl groups in the uranium layers. Therefore
a double uranium bridge is introduced (figure 4.1), which stabilizes the
Cs_U,O._ structure after folding the hexagonal uranium layers. Cs„U,.O.,
has a similar structure and forms a solic
elevated temperatures (see section 4.5).
has a similar structure and forms a solid solution with Cs.U.O.» at2 4 13
8.4. Uranium-uranium distances and uranium-oxygen bonding
Kovba [6] and Keller [7] suggested a connection between the uranium-oxygen
bonding type and the smallest uranium-uranium distance in a crystal struc-
ture. However, their assumptions were based on incomplete and sometimes
even erroneous data, as will be shown.
The diuranate structures were believed to contain perfect hexagonal
uranium layers. Then the smallest U-U distance can be calculated from the
unit cell parameters only [7]. Since monoclinic deformations for Na„U_07
[2,10], K2U"2O and R b ^ O j [chapter VII] and Cs2U2C>7 [chapter VI] have
been established by now, the smallest U-Ü distances can only be calculated
when the complete crystal structure or at least the metal positions are
known.
Keller reported the shortest U-U distance for hexagonal layers to
be 3.91 A. But on inspection of the cesium-uranate structures it is
found that Cs.U 0 ? contains U-U distances as small as 3.56. X
(figure 8.6c), whereas in the Cs.U ,0,, structure the U-U distance is
even 3.51 8. In table 8.3 the ranges of U-U distances in some typical
Table 8.3.
Ranges of uranium-uranium distances in some (pseudo)-hexagonal
uranium layers.
compound
Cs4U5O17
CS2Uy022
CS2U.5°46a-U-Oj, [20-22]
3-UO3 [23]
3.
3.
3.
3.
3.
range A
56 -
66 -
51 -
75 -
76 -
4.42
4.38
4.38
4.18
4.08
!*•• · Ί)
EP; i
If
';••• ;;>,'-7T$: • -Υ-..'•-! κ'ï4'";-;f -«ίΤ-'-; :ff .*• -;p^t0?M^ if ' \ "':
; '•* ^^JSg
f'
r: *,! • - • • . !
-85-
(pseudo)-hexagonal uranium layers are collected. Similar ranges can be
obtained from K ^ O ^ [chapter VII], K ^ O ^ [24] and Na2U2O7 [10].
For the tetragonal layer structures Keller reported the smallest
U-U distance to be 4.36 X. However, the tetragonal parts of the ot- and
B-Cs-ILO. uranium layers (figure 8.6a and 8.6b) contain distances of
4.18 X and 4.30 X, respectively.
y.,1%·,:·.
v
h ••
(a) (b)
. a.
Uranium-oxygen layers in a.-Cs^U2O? (a), $-Cs2U2O? (b) and Cs.UJ) (a).
For the numbering of the atoms see figures 6.3 and S.I (small spheres
designate uranium atoms, large spheres designate oxygen atoms). The
values in the figures are mutual uranium-uranium distances (in R).
Finally the shortest U-Ü distances in Na.UO,. and Li.UO are 4.64 X
and 4.46 X [13], in contradiction with the values given by Keller [7].
The 5.40 X Ü-U distance in the ISOL structure of a-Li,U0, [16] is much
smaller than the minimum 6.21 A for isolated-octahedra structures, re-
ported by Keller [7].
It can be concluded that the assumed connection between the shortest
U-U distance and the uranium-oxygen bonding type in alkali-uranate(VI)
ii-86-
'ΐ' I.
structures is less pronounced than suggested by Kovba [6] and Keller [7],
although of course in general the U-U distances in an ISOL structure are
greater than in the other five structure types.
From the values given and those listed in table 8.3 it is evident
that the ranges of U-U distances found in each of the types L , L ,
o > Cyo 0 an(
* ^ overlap. Therefore it is worthwhile to examine the
positions of the oxygen atoms in connection with the U-U distances. It
appears that the oxygen coordination polyhedra of the uranium atoms with
a short mutual U-U distance (3.5 - 3.7 A) share an edge, whereas for
longer U-U distances (4.3 - 4.5 A) a corner is shared. This fact is also
illustrated in figure 8.6.
8.5. The composition of the uranium layers
After the work of Zachariasen on the crystal structures of a-UO_ [273
and CaUO, [28] it has been believed for a long time that uranium(VI)
ions in hexagonal layers are coordinated by eight oxygen atoms normally.
Two of these oxygen atoms form the linear uranyl group, the other six
are arranged in a puckered ring perpendicular to the uranyl-group axis.
Such perfect hexagonal layers (figure 8.2a) have the composition
[(UC>2)0~], which can also be expressed by an O/Ur-ratio of 2.0. Layers
in which 0/Ur<2.0 were believed to contain statistical oxygen vacan-
cies [6,7,8,29,30,31].
Several investigators [9,10,20-24,32,33] have shown the latter
hypothesis to be wrong. Indeed many uranate(VI) structures have been
established, which contain (pseudo)-hexagonal layers with a variety of
compositions, the uranium(VI) ions being coordinated in the hexagonal
plane by four or five oxygen ions in an ordered way.
The variety of 0/Ur-ratios has been expressed in table 8.4. In the
column "layer notation" the layer structure has been summarized. For
example the notation Cs (U0,>) (U0_) Og in the case of Cs U 0 „ means
a layer structure built from sheets with two cesium atoms and two uranyl
groups and sheets consisting of five uranyl groups bonded in the layer
by eight oxygen atoms. Most of these layer notations have been derived
from the foregoing chapters or from the literature.
The layer notation of Cs-U,0 _ and Cs.U 0 ,, as listed in table 8.4,
needs some further explanation. In figure 8.7 a detailed picture of the
double uranium bridges in Cs U,0 and Cs_U,0 , is given. Since the U-U
».- - " -nimjlwnriVT\r~
I
I1 r
-87-
Oxygen-uranyl ratios in uranium layers.
compound
a-U3O
8 120]
β-UO [23]
CS2
U,5°46
CS2
U7°22
CS2
U4°13
CS4
U5°17
CS2
U2°7
M2
U7°22
M2
U4°13
M2
U2°7
layer notation
(u3o
3)o
5
Cs2(UO
2)5.(UO
2)
1 0O
1 6
Cs„(UO,)„ . (U0,),0a2 2 2 2 5 8
4[Cs,(U90~) . (U0„)._0 _]
Cs4 . (UO
2)
5O
7
Cs_ . (U0_)~0,
Cs_ . (U0„)0„
M2(UO
2)
2 . (UO
2)
5O
7
1 [ M 8 ( Ü O 2 ) 3 . ( U O 2 ) 1 3 O 2 0 ]
M2 . (UO2)O2
type
hex.
hex.
hex.
hex.
hex.
hex.
mixed
tetr.
hex.
hex.
hex.
tetr.
M/U **>
0.0
0.0
0.133
0.286
0.4
0.5
0.8
1.0
2.0
0.286
0.5
1.0
2.0
0/Ur +^
1.67
1.67
1.60
1.60
1.75
1.70
1.40
1.50
2.00
1.60
1.54
1.50
2.00
*) Μ = K,Rb**)
Μ = K,Rb,Cst)
in the uranium layer
distance in this double uranium bridge is about 3.5 A [chapter IV] the
oxygen polyhedra share an edge. Together with the oxygen atoms of the
(pseudo)-hexagonal layers the uranium atoms in the bridge can form
stable coordinations. From this structure model the layer notation in
table 8.4 can be derived.
From the column "layer notation" the 0/Ur-ratios of the layers can
easily be calculated. From table 8.4 a relationship between the layer-
composition, i.e. the 0/Ur-ratio, and the type of the uranium layer is
not found. Apparently the radius of the metal ion and its "concentration"
play a more important role than the 0/Ur-ratio, as was'stated earlier
in section 8.3, rule 2. A relationship between the M/U-ratio and the
composition of the layer is not found either.
As a next point the ranges of U-U distances in the layers, as listed
in table 8.3 can be considered. Confining ourselves to the (pseudo)-
hexagonal uranium layers, a relationship between the U-U distances and
the 0/Ur-ratios is not found.
R ••'••',·
Μ- : -j
i.-: iS
if. • «:
•·§
-88-
Figure 8.7.
Bonding in the double uranium bridges in the CaJJ.O.^ and CsJJO1R
structures (compare to figure 4.1). The small black spheres form the
double uranium bridges; the large open spheres designate oxygen atoms.
The black uranium atoms can form wcanyl bonding with the uranyl oxygen
atoms bonded to the shaded vacanivm atoms in the (pseudo)-hexagonal
layers.
Therefore it must be concluded that it is not possible to formulate
a general rule concerning the coordination of uranium by oxygen in re-
lation to the layer composition.
Overlooking the (pseudo)-hexagonal layers of Cs,U-O._, Cs.UyU-- and
oc-U~0„, as reproduced in figure 8.8, an increase of 4-coordinated uranium
atoms (in the layer plane) and 3-coordinated holes seems to be met when
the O/Ur-ratio decreases. However, this tendency does not hold when
the (pseudo)-hexagonal layers of a-U_O and g-UO_ are compared (figuresJo j
8.8c and 8.8d): the layers in these compounds have the same composition
but the coordination of uranium is different, whereas the mean U-U dis-
tances in these layers are almost equal, as follows from table 8.3. The
same conclusion can be drawn upon comparison of the layers in a-U_O„ and
6-U3Og [21,22].
1ti 4
-89-
(α)
Figure 8.8.
Oxygen coordinations in (pseudo)-hexagonal „\-mium layers in Cs UJ) (a),Cs2
U7°22 ^>
a~
U3°8 ^
an^ ^~
U03 ^> · The >.vts designate the uranium
positions in the layers, the thick lines give 'he unit cells of the
structures; 3-coordinated "holes" in the layers have been shaded.
ι ;:
f Λ'I " •' ·
Esfe
-90-
8.6. Uranium layers in Cs^U.O., and CsJ Ο ,
In the (pseudo)-hexagonal layers, which were studied in section 8.5,
sixfold and sevenfold coordinations of uranium by oxygen were always
found. In the a-U,08 uranium layers, as shown in figure 8.8c, all space
between the uranium atoms has been occupied by oxygen atoms, all uranium
atoms being sevenfold (5+2) coordinated. This suggests that the O/Ur-
ratio 1.67 of ot-U.O- is a limit for (pseudo)-hexagonal uranium layers.
5 o
However, the 0/Ur-ratios in CsJJ.O,- and Cs.U.O,, are 1.70 and 1.75,
ί ή 1J /3 1ο
respectively.
The assumption that eightfold coordinations (6+2) are present in
these hexagonal layers is obvious, but there are more ways possible to
explain the high 0/Ur-ratios in Cs2U,0 _ and Cs U O .
Firstly the interlayer might contain oxygen atoms in addition to
the double uranium bridges and cesium atoms, but this has never been ob-
served in any uranate structure.
Secondly the hexagonal uranium layers might be uranium-deficient.A similar fact has been observed in the a-U0_ structure, which is a
uranium-deficient a-U-0 structure [9]. As a consequence of the uranium
deficiency and because of the charge neutrality also some oxygen must be
missing in the Cs.U.O.- structure. In addition the Cs/U-ratio should be
somewhat higher than 0.5. The uranium deficiency in the (pseudo)-hexagonal
layers is suggested by the chemical relation between Cs.U.O and Cs_U,0]2
according to the equilibrium reaction
which was discussed in section 2.4.2. Although this chemical relation
between Cs„U,0 „ and Cs-U.O.- does not necessarily imply a structural
relation between these two compounds, it is striking that in the
γ-Cs.ü.O . structure also hexagonal uranium layers exist, in which part
of the uranium atoms is missing, as indicated in figure 8.9. An increase
of the Cs/U-ratio in Cs.U.O.j has been observed (see next paragraph), but
carries little weight because of the low accuracy of the cesium analysis.
Finally one could imagine the (pseudo)-hexagonal uranium layers in
Cs.U.CL, and Cs_U,0., to have an O/Ur-ratio of J.67 by oxygen deficiency,
which implies that part of the uranium atoms is in the pentavalent state.
This possibility is supported by the colours of Cs„U,0._ and Cs-U.O.,:
they are much darker (yellow-brown, sometimes even deep-red) than the
colours of the other cesium uranates. Supposing an 0/Ur-ratio of 1.67 mm1111
-91-
VI V VI V
in the layers the formulae Cs6U
) ; )U
2O
4 7 and Cs
gU
J7U 0
5 g are derived
for "Cs-Uj-O ," and "Cs^U.O " respectively. A chemical analysis
of a Cs„U,0 sample (see table 8.5) led to the formula
Cs„uY* ,Ίϋ
η n,0,_ _., which implies that this explanation for the high
O/Ur-ratios should be rejected, where the accuracy of ••he cesium
analysis is over-estimated.
Table 8.5.
Chemical analysis of CSJJ\0
element
Cs
U(tot)
U(IV)
content (%)
18.78
66.57
0.277
. 1 :'-
Figure 8.9.
Hexagonal uranium layers in y-CspU 0 „. The plane drawn in (b)
} has
been indicated in cube (a), which should be compared to figure 3.2a.
8.7. Influence of the metal radius on the interplanar distance in layer
structures of uranates(VI)
The influence of the alkali-metal ion is clear from the interlayer dis-
tances of the crystal structures, i.e. the distance between the suc-
ceeding hexagonal or tetragonal uranium layers. In figure 8.10 the
Interlayer distance (A)
0]
«5
roCo
Cl Cb
01
g·
s3
«9
I
f!; 'ft
-93-
interlayer distances of the alkali uranates(VI) are collected. In general
the interlayer distance increases with the alkali-metal ion radius, as
could be expected.
In the case of potassium and rubidium the interlayer distance de-
creases upon introduction of additional alkali ions in the interlayer·
This might be understood by considering the negative charge on the oxygen
atoms and the positive charge on the alkali ions. Since potassium and
oxygen have similar radii, there will be an increase of interlayer dis-
tance on substitution of an alkali ion by an uranyl group, because in
the former case there is alkali-metal/oxygen attraction and in the latter
case there is oxygen/oxygen repulsion.
In the case of cesium the metal radius is so large that the inter-
layer distance will depend largely upon the cesium ion. In Cs_U 0 , and
Cs U.O., the double bridges cause a very large int^rlayer distance.
Since the penetration of alkali ions in the tetragonal motif is
greater than for the hexagonal motif, the interlayer distance in a- and
B-Cs„U„O7 is rather short.
This tentative structure description is illustrated in figure 8.11.
From figure 8.10 it also follows that at low M/U-ratios the uranate(VI)
interlayer distances approach that of β-ÜO..
Figure 8.11.
Schematic view of the interlayers of some alkali uranates(VI). The blaakbars designate the (pseudo)-hexagonal uranism layers, whereas the openbars designate the (pseudo)-tetragonal uram'-.-n layers. The two concentriccircles denote a projection of the uranyl groups in the interlayers. Thelarge open spheres denote cesium and potassium atoms3 respectively. Inthe figure a penetration of the alkali ion into the tetragonal layer issuggested, in contrast to the hexagonal layer. The double bridges makethe situation in Cs^U,£>]•£ and Cs^U^O-jg rather camples.
Mw·
-94-
r ι
i-kaSI ν
8.8. Uranates(V) with the perovskite structure
In the foregoing sections of this chapter much attention has been paid
to the crystal structures of alkali uranates, in which uranium is in the
hexavalent state. In this section some alkali uranates(V) will be con-
sidered.
In the introduction of this chapter the compounds LiUCL and NaUO»
have been mentioned. Together with KUO_ and RbUO- (section 7.4) a series
is formed, in which the influence of the alkali-metal ion radius can be
studied. A compound CsUO- has not been found, neither in this investiga-
tion (chapter II) nor by Kemmler-Sack et al. [5],
The crystal structure of LiUO_ is rhombohedral, isostructural with
LiNbO [34], The compounds NaUO,, KUO. and RbUO, have perovskite-type
structures. The structures of KUO and RbUO- are cubic (section 7.4)
whereas the NaUO structure is orthorhombically deformed [35].
Spitsyn et al. [36] and Kemmler-Sack [5] have discussed the use of
the Goldschmidt criterion [37] for uranate structures. The Goldschmidt
tolerance factor t for MUO. compounds has to be taken as
+ r ,2-
l u
5 + o
1-
in which r stands for the respective ion radii. Perovskite structures
are stable when t lies in the range 0.9 - 1.1 [38]. Other investigators
[39] have pointed out that at the limits of this range results are
rather unreliable.
However the radii of the respective ions are not accurately known.
Since a long time the Pauling radii [40] have been in use, but recently
Narayan and Ramaseshan [41] have published values which are much greater
than the alkali Pauling radii. In addition the radius of the uranium(V)
ion is not known accurately [5]. Therefore the Goldschmidt criterion can
hardly be applied in case of the MUO- series.
Another method to discuss the possible existence of the CsUO,
perovskite structure is to estimate its unit cell edge, whereupon the
Cs-0 distance can be calculated and compared to literature values.
The perovskite cell parameter of CsUO- can be estimated from the
values for KUO, and RbUO-, as given in section 7.4. In figure 8.12a
a linear extrapolation of these cell constants is given, based on the
1 >
'Λ,
1i '-''ή ,
3 ..
H'\.:<!•'.
m-95-
«t 435
(a)A
Figure 8.12.
Estimate of the oubie cell parameter of the hypothetic CsUO„
perovskite, according to the metal ion radius (a) and the cell
parameter of the tetragonal MJJO. structures (b).
Pauling radii of Κ ,Rb and Cs . Since the configuration in the tetra-
gonal layers in M.UO, compounds (M = K,Rb,Cs) is very similar to the
configuration in the cubic perovskite structures, the. unit cell para-
meter of CsUO_ can also be estimated by comparison of the tetragonal
cell constants of M.UO,, as is shown in figure 8.12b.
From both estimates the unit cell parameter for CsUO_ is found to
be 4.38(3) X. From this a Cs-0 distance of 3.10(2) % can be calculated.
From the International Tables [42] a Cs-0 range 3.28 - 3.42 X is found
for 12-coordinated cesium atoms. Therefore the conclusion must be that
a perovskite structure with formula CsU0_ is not stable.
References
[1] P.M. de Wolff, J. Appl. Cryst., y 108 (1968).
[2] E.H.P. Cordfunke and B.O. Loopstra, J. inorg. nucl. Chem., 33_,
2427 (1971).
[3] L.M. Kovba, Russ. J. inorg. Chem., J_6, 1639 (1971).
[4] J. Hauck, J. inorg. nucl. Chem., 3i6, 2291 (1974).
[5] S. Kemmler-Sac.k and W. Rüdorff, Ζ. anorg. allg. Chem., 354,
255 (1967).
11:Η
4
at.*ü:m
L
-96-
[6] L.M. Kovba, Izvest. Vyssikh. Ucheb. Zavedinii, Khim. i Khim.
Tekhnol., 2» 219 (I960); see Chem. Abstr., 54, no. 21900 (1960).
[7] C. Keiler, Habilitationsschrift, T.H. Fredericiana, Karlsruhe,
p.135, (1964); also KFK-225, Karlsruhe, (1960).
[8] J. Prigent and J. Lucas, Bull. Soc. Chim. France, 1129 (1965).
[9] C. Greaves and B.E.F. Fender, Acta Cryst., B28, 3609 (1972).
[10] L.M. Kovba, Radiokhimiya, U^, 727 (1972).
[II] L.M. Kovba, E.A. Ippolitova, Yu,P. Simanov and V.I. Spitsyn,
Russ. J. inorg. Cbsm., 21* 275 (1961).
[12] L.M. Kovba, Vestn. Mosk. Univ., Khim. Ser., \2_, 489 (1971).
[13] H.R. Hcikstra and S. Siegel, J. inorg. nucl. Chem., 2£, 693 (1964).
[14] H.R. Hoekstra, J. inorg. nucl. Chem., 2J_, 801 (1965).
[15] D.G. Kepert, Thesis, University of Melbourne; cited in [13] and [14].
[16] J. Hauck, Z. Naturforsch., 28b, 215 (1973).
[17] H.M. Rietveld, Acta Cryst., 20, 508 (1966).
[18] C. Greaves, A.K, Cheetham and B.E.F. Fender, Inorg. Chem., J_2,
3003 (1973).
[19] L.M. Kovba, Zh. Strukt. Khimii, J^, 4 5 8 ('972).
[20] B.O. Loopstra, Acta Cryst., V^, 651 (1964).
[21] B.O. Loopstra, Acta Cryst., B26, 656 (1970).
[22] B.O. Loopstra, J. Appl. Cryst., J}. 94 (1970).
[23] P.C. Debets, Acta Cryst., 2JU 589 (1966).
[24] L.M. Kovba, Zh. Strukt. Khimii, 3, 256 (1972).
[25] E.H.P. Cordfunke, A.B. van Egmond and G, van Voorst, J. inorg. nucl.
Chem., 37, 1433 (1975).
[26] L.M. Kovba and V.I. Trunova, Radiokhimiya, JTJ, 7 7 3 ('973).
[27] W.H. Zachariasen, Acta Cryst., ±, 265 (1948).
[28] W.H. Zachariasen, Acta Cryst., JU 281 (1948).
L29] L.M. Kovba, Radiokhimiya, JL2, 522 (1970).
(-.Trifj-i ,ΐ,^ϊνΖ*'-
'. · 'i
-97-
[30] L.M. Kovba, Ι.Α. Murav'eva and A.S. Orlova, Radiokhimiya, _[6_,
648 (1974).
[31] L.M. Kovba, E.A. Ippolitova, Yu.P. Simanov and V.I. Spitsyn,
Dokl. Akad. Nauk. SSSR, 20, 1042 (1958).
[32] A.F. Andresen, Acta Cryst., U_, 612 (1958).
[33] B. Chodura and J. Maly, Second Int. Conf. Peaceful Uses Atomic
Energy, 28, 223 (1958).
[34] S. Kemmler, Z. anorg. allg. Chem., 338, 9 (1965).
[35] S.F. Bartramm and R.E. Fryxell, J. inorg. nucl. Chem., 32, 3701
(1970).
[36] V.I. Spitsyn, E.A. Ippolitova, Yu.P. Simanov, L.M. Kovba, in
Investigations in the Field of Uranium Chemistry, see chapter II,
ref. [2], p.4-16.
[37] V.M. Goldschmidt, Srk. Akad. Oslo, I. Mat. nat. KI., 2_, 97 (1926).
[38] W.H. Zachariasen, Srk. Akad. Oslo, I. Mat. nat. KI., U, (1928).
[39] D. Balz and K. Plieth, Z. Elektrochemie, 59, 545 (1955).
[40] L. Pauling, Proc. Roy. Soc, London A114, 181 (1927).
[41] R. Narayan and S. Ramaseshan, J. Phys. Chem. Solids, 21» 395 (1976).
[42] International Tables for X-ray Crystallography, Birmingham (1962).
MNBBau*^r
3 !
APPENDICES
-98-
(BStf
Remarks
I. OBS and CALC
OBS and CALC
2 . Q-OBS
3 . I/IO
. andj
in Α-appendices are F . and F .K K obs calc
(see section 1.2.1)·
in B-appendices are summations of n. F2 J
η. F
c aj
c : (see section 1.2.I).
Negative OBS means no contribution of intensity
to the least-squares structure refinement.
in B- and C-appendices are observed Q-values.
in B- and C-appendices are intensities of powder
diffraction lines relative to the strongest line
(= 100).
I!
1!
····· x-BA» DATA OF ALKALI UPANATES
APPFNOIX
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BB
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c?
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Clfc
COMPOUND
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SAMENVATTING
Dit proefschrift beschrijft onderzoekingen aan de kristalstrukturen van
cesiumuranaten met behulp van voornamelijk Rontgendiffractie.
Hoofdstuk I noemt twee redenen voor onderzoek van cesiumuranaten.
Deze verbindingen spelen een belar.grijke rol in het gedrag van splijt-
stofstaven in kernreaktoren. Daart.aast is het interessant om na te gaan
welke de invloed van de alkali-ionstrueï is in de kristalstrukturen van
de alkali-uranaten, (uraanzouten van de eenwaardige metalen lithium,
natrium, kalium, rubidium en cesium).
In hoofdstuk II wordt aangegeven welke cesiumuranaten stabiel zijn
in het Cs-U-0 systeem, zowel in lucht als bij lage zuurstofdrukken. Uit
deze fasenstudie blijkt dat de verbinding Cs.U.O „ de grootste rol zal
spelen in de splijtstofstaven van snelle kweekreaktoren.
De drie kristalstrukturen van Cs-U,O.„ worden beschreven in hoofd-
stuk III, Het zwellen van de splijtstof bij het ontstaan van "cesium-
uranaat" blijkt verklaard te kunnen worden door het verschil in soorte-
lijke dichtheden van Cs-U.O.. and Ü0-.
In de hoofdstukken IV, V en vi worden de kristalstrukturen van de
zeswaardige cesiumuranaten, te weten Cs„UO,, Cs^U-O-, Cs,U,O._, CSjU.O.,,
Cs„U_O.g, Cs„UyO„2 en Cs.U.-O,,, beschreven. Slechts van twee verbindingen,
Cs-U^O.- en Cs„U,O.,,, kan dit gedaan worden met behulp van Rontgen-
diffractie aan éénkristallen; van de overige wordt de kristalstruktuur
bepaald met behulp van Rontgendiffractie aan poederpreparaten. In go·, al
van Cs„U_07 (hoofdstuk VI) wordt tevens gebruik gemaakt van neutronen-
en elektronenciffractie.
Hoofdstuk VII beschrijft de ' alium- en T.'ubidiumuranaat systemen.
Omdat de literatuur veel tegenstrijdigheden aangaande daze uranaten
bevat, zijn de K-U-0 en Rb-ü-0 systemen onderzocht, zowel in lucht JIS
in inerte atmosfeer. Beide systemen zijn aanmerkelijk eenvoudiger dan
het Cs-U-0 systeem. Alle kaliumuranaten zijn volledig ifostructureel
met de desbetreffende rubidiumuranaten.
In hoofdstuk VIII wordt tenslotte een vergelijk getrokken tussen
de alkali-uranaten. De kristalstrukturen van de verschillende uranaten
blijken te kunnen worden beschreven op basis van een zestal struktuur-
karakceristieken; (pseudo) hexagonale uraniumlagen, (pseudo) tetragonale
uraniumlagen, ketens met samenstelling (U02)0„, ketens met samenstelling
(U0^)0, geïsoleerde uraanomringingen en driedimensionale uraan-zuurstof
Liaari—Wf f' rs"i •"•jte
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roosters. De grootte en de "concentratie" van het alkali-metaal ion
blijken doorslaggevend voor de gevonden kristalstruktuur. Een duidelijk
verband tussen de omringing van uranium door zuurstof en de samenstel-
ling van de uraniumlagen (uitgedrukt in de verhouding zuurstof/uranyl
groepen) is niet aan te geven. De grootte van het alkali-metaal ion
wordt ook teruggevonden in de afstand tussen de diverse uraniumlager. in
de 1agenstrukturen. Tenslotte wordt verduidelijkt waarom een verbinding
CslIO- met perovskietstruktuur niet gevonden wordt in het cesiumuranaat
systeem.
ils!
- - Ö
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CÜRRICULUM VITAE
Andrê van Egmond, geboren 19 juni 1950 te Hengelo (0), behaalde het
HBS-b diploma aan het Ichthus College te Enschede in 1967. In datzelfde
jaar begon hij sijn studie aan de Technische Hogeschool Twente. Het
baccalaureaatsexamen chemische technologie legde hij af in januari 1971.
In juni 1973 werd het ingenieursdiploma (richting chemische fysica)
behaald. Tijdens zijn afstudeerperiode verbleef hij gedurende zes maanden
in het IBM Research Laboratory te San JosS (Calif.), waar enkele labora-
toriumautomatiseringssystemen bestudeerd werden.
Sinds augustus 1973 is hij als gast-medewerker in dienst van het
Reactor Centrum Nederland, waar het onderzoek, beschreven in dit proef-
schrift, werd verricht.
