Submitted to “Advances in X-ray Analysis” - Proceedings of the 45th Annual X-ray
Conference, Denver, Colorado, USA.
Solid Solution Behaviour of Synthetic Monazite and Xenotime from Structure
Refinement of Powder Data.
by BOB VAN EMDEN
Special Research Centre for Advanced Mineral and Materials Processing, Chemistry
Department, University of Western Australia, Nedlands, W.A., 6907, Australia.
MIKE R. THORNBER,
Special Research Centre for Advanced Mineral and Materials Processing. CSIRO, Division of
Minerals, Waterford, W.A., 6102, Australia.
JIM GRAHAM AND FRANK J. LINCOLN.
Special Research Centre for Advanced Mineral and Materials Processing, Chemistry
Department, University of Western Australia, Nedlands, W.A., 6907, Australia.
All correspondence to be addressed to :
Dr. F.J. Lincoln
Special Research Centre for Advanced Mineral and Materials Processing
Department of Chemistry, University of Western Australia
Nedlands, W.A., 6907
Australia
Telephone : (619)3803142
Facsimile : (619)3801116
Email : [email protected]
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ISSN 1097-0002, Advances in X-ray Analysis, Volume 40
ABSTRACT
In order to study the complex substitution behaviour observed in natural samples, synthetic structural
analogues of the rare earth element (REE) orthophosphate minerals monazite and xenotime have been
prepared. Compounds of the type: (Nd,Yi_,)POb , (La,Yi.,)PO4 , (Sm,Yi_,)POd and (Nd,Ybi_,)PO4 have
been prepared both by precipitation and solid state methods and then heated at temperatures of 1000,
1200 and 1500 “C. Structure refinement of powder X-ray data, using Rietveld full profile methods, has
been carried out on these mixed light- and heavy-REE phosphates to obtain cell parameters and relative
proportions of co-existing phases. The lever law was used to estimate the extent of heavy-REE
substitution in synthetic light-REE-containing monazite phases and light-REE substitution in heavy-
REE-containing xenotime phases. In general, the larger the difference in radii of the two REEs in each
phase, the lower the extent of REE substitution, A higher level of both heavy-REE substitution in
monazite phases and light-REE substitution in xenotime phases was found in preparations heated to
1500 OC. Xenotime phases, containing Y in combination with smaller light-REEs, which have been
heated at 15OO”C, show some evidence of REE ordering, but this behaviour has not yet been well
characterised.
INTRODUCTION
Monazite, a Light Rare Earth Element (LREE) phosphate mineral, crystallises in the monazite
structure type with a monoclinic unit cell and a space group P2l/n (no. 14) (Ni et al., 1995). The REE
atom is in nine-fold coordination with the oxygen atoms of slightly distorted phosphate tetrahedra. The
Heavy REE (HREE) phosphate mineral, xenotime, crystallises in the more dense zircon structure type
with a tetragonal unit cell and a space group 14l/amd (no. 141) (Kristanovic, 1965). The REE atom is
in eight-fold coordination with the oxygen atoms of phosphate tetrahedra. The effective ionic radius (r)
of the REE atom determines which of the two structure types are formed. The larger radii LREEs
crystallise together with phosphate in the monazite structure, whilst the smaller radii HREEs crystallise
in the zircon structure (Miyawaki and Nakai, 1993). We have prepared synthetic structural analogues
of these minerals containing different combinations of light and heavy REEs to determine the
composition range of the structural boundary, and, decide if any REE ordering occurs within the
phases. The crystal chemical limits of HREE substitution in monazite and of LREE in xenotime have
been determined by use of powder X-ray diffraction techniques. The Rietveld method was used to
obtain the unit cell parameters and relative proportion of each phase within a particular compound.
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Copyright (C) JCPDS-International Centre for Diffraction Data 1997ISSN 1097-0002, Advances in X-ray Analysis, Volume 40
The synthesis temperature was varied to determine its influence on the level of REE
substitution in each of the resultant phases.
EXPERIMENTAL METHODS
i/Synthesis
The synthetic phases were prepared by both precipitation from solution and solid state
reaction. The precipitation procedure was used to prepare compounds of the type [NdxY(l_x)]PO4 (for
x < 1) by heating the corresponding hydrated compounds formed by precipitation from solution. The
procedure involved adding phosphoric acid to a stirred solution of Nd3+ and Y3+ ions (combined
concentration of 0.05 moles / dm3) in various proportions, so that P / (Nd + Y) = 20. The precipitate
that formed after pH adjustment and constant stirring (2 days at 40°C) was collected, washed with
water and then dried (105’C for 2 days). A portion of each precipitate was then heated to temperatures
up to 1OOO’C for 48 hours.
Synthesis via solid state reaction was used to prepare several compounds of [NdxY(l_x)]P04
and all other LREE - HREE compounds as this method was more convenient. The solid state reaction
involved mixing the correct proportion of light and heavy REE203 with (NH4)2HP04, pelletising and
firing at 9OO’C (16 hours), regrinding, pelletising and then firing at 12OO’C (24 hours) following a
procedure similar to that used by McCarthy et al. (1978). Samples fired to higher temperatures were
again reground and pelletised.
ii/ Powder X-ray diffraction and SEA!
Samples for powder X-ray diffraction (XRD) analysis were hand ground by agate mortar and
pestle for a minimum of 10 minutes and mounted so as to minimise preferred orientation effects. Scans
were conducted at room temperature on a Siemens D5000 diffractometer with Cu Ka radiation at 40
kV and 35 mA. Initial phase identification was performed using the Diffrac program and ppdsm
search-match software. Typical Rietveld scan conditions from which cell data were obtained were, a
Bragg range of 14 - 80° at a step width of O.O4O, with 2.5 seconds count time. These conditions are
generally considered to be sufficient to obtain representative estimated standard deviations in cell
parameters (Hill, 1995).
The Rietveld program used is based on that by Wiles and Young (1981). Most refinements
were limited to the refinement of scale factors, zero point, four background coefficients, cell
parameters, peak widths, peak asymmetry and peak shape coefficients. Scanning electron microscopy
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(SEM) was performed on uncoated samples on a JEOL JSM 5800 low vacuum SEM with EDAX
detector and Oxford ISIS image software located at CSIRO, Waterford, WA.
RESULTS AND DISCUSSION
i/ [IVdxY(l_x)]P04 type compounds
A number of compounds containing various amounts of Nd and Y together with phosphate
were prepared by precipitation and heated to 1000°C. The unit cell parameters of each phase formed
were determined by Rietveld analysis of the powder XRD patterns. Those compounds containing less
than 30 mole % Y formed single phase monazite. Compounds containing 40 mole % Y or more formed
either single phase xenotime or both phases. The level of Y 3-t held in the monazite phase heated at
1000°C is then between 30 and 40 mole%. Y is not incorporated in the monazite structure as efficiently
as expected from the average r (r-A) of REE3+, as otherwise up to 6.5 %Y should be held in the
structure.
The unit cell data for a number of monazite phases in the range of 0 to 30% Y are shown in
Table 1. The cell parameters a, b and c for each phase are slightly larger than expected from their
rA{ REE3+}. The unit cell volumes of the substituted phases are significantly larger than that expected
for ideal solid solution behaviour. This difference in unit cell volume is shown in Figure 1 which
compares the unit cell volumes of the substituted phases to those obtained for prepared samples of Nd
and Sm phosphate. It appears that the concept of rA{ REE3+} can not be reliably used to predict the
unit cell parameters of mixed LREE - HREE monazite type phases.
The unit cell data of phases in several compounds of [NdxY(l_x)]P04 for which x is between
0 and 0.15 were determined from Rietveld analysis of powder X-ray data. They indicate that the limit
of Nd3+ substitution in the xenotime structure YP04 is between 10 and 15% as the cell parameters of
the xenotime phase in two phase mixtures are only slightly larger than those of single phase xenotime
containing 10 % Nd. Up to 23% Nd 3+ should be held in the xenotime phase YP04 if it formed a
perfect solid solution.
Table 2 shows the unit cell data of phases in several compounds of [Nd,Y(l_,)]PO4 for which
both a monazite and a xenotime phase formed. The unit cell data of the monazite and xenotime phase in
each of the compounds are nearly identical, indicating that phases of similar composition are formed. A
state of, at least, temporary equilibrium appears to exist at this temperature.
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Table 1. Unit cell parameters of monazite type precipitate phases in compounds of the type
[NdxY(l_x)]P04 heated to lOOO’C, with estimated standard deviations (esd’s) given in parenthesis.
Atom % Y
in REE site
0
rAIX{REE3+] (A), Cell Parameters (A) Profile Fit
Cell Vol. (A3)
1.163 a 6.7448(2)
b 6.9552(2) RB = 2.68.
Vc = 292.368 c 6.4140(2) GofF = 3.08.
b 103.669(2)
5
1.1586
Vc = 29 1.347
a 6.7363(5)
b 6.9495(5)
c 6.4058(5)
b 103.701(5)
RB = 7.23.
GofF = 2.63.
10
1.1542
Vc = 291.190
a 6.7345(6)
b 6.9471(6)
c 6.4070(5)
b 103.728(6)
RB = 3.03.
GofF = 1.42.
20
1.1454
Vc = 289.216
a 6.7187(7)
b 6.9290(7)
c 6.3962(6)
b 103.765(6)
RB = 5.65.
GofF = 2.03.
30
1.1366
Vc = 287.228
a 6.7030(g)
b 6.9 109(8)
c 6.3850(7)
b 103.809(7)
RB = 8.32.
GofF = 2.69.
Notes : RB : Bragg R factor (%)
GofF : Goodness of fit
Vc : Unit cell volume.
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291
289
287
1.13 1.14 1.15 1.16
rALx (REE 3+J (A)
Figure 1. Unit cell volume of single phase monazite type [(NdxY(l_x)]PO4 phases against the
rag{ REE3+}.
ii/ LaxY(i-x)lPO4 type CO~~OUF~~S
The unit cell data of phases in several compounds of [LaxY(1_x)]P04 were determined. The
extent of Y3+ substitution in LaP04 is considerably less than that in NdP04 and is limited to less than
20 mole % Y3+ at 1200°C. This is far below the 80 mole % Y substitution expected from r-A{ REE3+}
considerations. The large size difference between the La 3+ and Y3+ ion (r”{La3+} = 1.216 ‘4 vs
1.075 A for r”{Y3+}; Shannon, 1976) appears to be difficult for the monazite structure to tolerate.
The cell parameters of the xenotime phase in [LaxY(1_x)]P04 compounds were found to be
only slightly larger than those of YP04, indicating that YP04 can only substitute a few mole % of
La3+. The large size of the La3+ ion (rIX{La3+} = 1.216 8, vs 1.163 A for rIX{Nd3+}) severely
restricts its substitution in the xenotime phase.
iii/ [SmxY(l _X)]P04 type compounds
The unit cell data of phases in several compounds of the type [SmxY(1_x)]P04 were
determined. The cell parameters of the monazite phase in the two phase compositions of (Sm.gY.g)P04
and (Sm.25Y.75)P04 are only slightly smaller than that of the single monazite phase of composition
(Sm.75Y.25)P04. The limit of Y 3+ substitution in SmPO4 at 1200°C is then a little greater than 25
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mole %. This REE composition has an I-A{REE~+} close to the r{ Gd3+} (rAIx = 1.118 A vs 1.107 A
for r”{ Gd3+}) - the smallest single REE ion to exist in the monazite structure (Ni et. al., 1995).
Table 2. Unit cell data of several [NdxY(l_x)]P04 compounds synthesised by solid state reaction at
1200°C which contain both monazite and xenotime phases (esd’s in parenthesis).
Compound, Monazite phase cell Xenotime phase cell Relative proportion
rA{ REE3+} (A) parameters (A) parameters (A) (wt.%)
tNdo.2oYo.8oY’O4 FIT = 1.037
lJX = 1.093.
Vc M = 285.391
Wo.25Yo.75PQ rvlI1 = 1.042
,1X = 1.097.
Vc M = 285.411
Wo.3oYo.7oY’O4 rvlI1 = 1.046
,1X = 1.101.
Vc M = 285.7 10
WosoYosoW4 rvlI1 = 1.064
lJX = 1.119.
Vc M = 285.838
a 6.6897(8)
b 6.8926(7)
c 6.3749(6)
b 103.855(11)
a 6.6893(7)
b 6.8934(6)
c 6.3753(5)
b 103.873(9)
a 6.6917(7)
b 6.8956(7)
c 6.3770(6)
b 103.842(9)
a 6.6927( 10)
b 6.8972( 10)
c 6.3779(8)
b 103.860( 10)
a 6.9025(4)
c 6.0370(3)
Vc X = 287.630
a 6.9016(3)
c 6.0365(3)
Vc X = 287.53 1
a 6.9024(4)
c 6.0362(4)
Vc X = 287.583
a 6.9037( 11)
c 6.0380( 12)
Vc X = 287.778
24.4(0.3) M,
74.6(0.6) X.
26.2(0.8) M,
73.8(1.0) X.
36.7(0.4) M,
63.3(0.5) X.
65.4( 1.2) M,
34.6(0.9) X.
Notes : VcM : Unit cell volume of monazite phase
VcX : Unit cell volume of xenotime phase.
The composition of the xenotime phase in two phase [SmxY(l-x)]P04 preparations is
estimated to be (Sm_l5Y.85)P04 from the composition of the monazite phase and the relative
proportion of each phase using the lever rule. This REE composition has an rA{REE3+} close to that
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of Tb3+ (rAVnl = 1.028 A vs 1.040 8, for r v111{ Tb3+}) - the largest single REE ion to exist in the
xenotime structure (Ni et. al., 1995).
iv/ [NdxYb(l_x)]P04 type compounds
The presence of two phases for the composition (Nd.75yb.25)PO4, and the closeness of the
xenotime phase cell parameters to those of single phase (Nd.gOYb.lO)P04 (see Table 3) suggests that
the amount of substitution of the small radii HREE ion Yb3+ (rvIn{Yb3+} = 0.985 A vs 1.019 8, for
rV1R{Y3+}) in monazite phase NdP04 is considerably lower than for Y3+. Lever rule estimations on
several of the two phase containing [NdxYb(1_x)]P04 compounds limit the substitution of Yb3+ to 12
mole % at 12OO’C. The large size difference between Nd3+ and Yb3+ restricts Yb3+ entry into the
monazite structure.
The level of Nd3+ substitution in xenotime phase YbP04 is limited to approximately 6 mole %
at 1200°C. This level of substitution is considerably lower than the 12 mole % substitution limit found
for Nd3+ in YP04.
v/ Injluence of temperature
Samples of (Nd.gYb.g)P04 and (La_6Y_4)PO4 were heated to temperatures of 1500°C and
1590°C to determine the change in REE substitution behaviour with temperature. The unit cell lengths
of the monazite phase in each compound decreased when the firing temperature was increased from
1200°C to 1590°C. This change in cell lengths indicates that a greater proportion of HREE occurs in
the monazite phase heated at the higher temperature. The unit cell parameters of monazite and
xenotime phases in (La.6Ye4)P04 heated to 12OO”C, 1500°C and 1590°C are shown in Table 4. The
unit cell lengths of the monazite phase and the proportion of the xenotime phase decrease upon heating
from 1200°C to 1590°C. (La.gY.q)P04 is in fact single phase monazite at 1590°C (see Table 4).
The rAM { REE3+} in (La.6Ya4)P04 is close to the r”{Nd3+} (1.160 A vs. 1.163 8, for
Nd3+). The unit cell lengths (of [Lae6Y_4]P04) a and b are slightly smaller whilst the c - length is
significantly larger than those of NdP04 (see Table 4). A preferential elongation of the alternating
REE polyhedra - phosphate tetrahedra chains in the monazite structure which are aligned parallel to the
c-axis must occur. Each chain is connected to five others through oxygen atoms which (when joined)
describe a pentagonal plane (Mullica et al., 1985). This type of linkage locks the structure and explains
why there is less flexibility between chains than within.
The cell parameters of the xenotime phase in (Nd.gYb.g)PO4 and (La.gY.q)P04 compounds
show no significant change with increasing temperature (see Table 4 for example). The extent of LREE
substitution in the xenotime phase of (Nde5Yb.5)P04 and (La_6Y_4)PO4 compounds is unaffected by
temperature. The higher density and symmetry of the zircon structure type in which xenotime phases
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crystallise (Milligan et al., 1983) appears to place greater REE size restrictions than does the monazite
structure.
Table 3. Unit cell data of synthetic monazite and xenotime phases in compounds of the type :
[NdxYb(l_x)]P04 heated to 1200°C, with esd’s in parenthesis.
Compound, Monazite phase cell Xenotime phase cell Relative proportion
r~{ REE3+} (A) parameters (A) parameters (A) (wt. %)
NdPO4
rvlI1 = 1.109
rIx = 1.163.
V, M = 292.198
a 6.7428( 1)
b 6.9574( 1)
c 6.4103( 1)
b 103.675( 1)
100 M.
YbP04
rvlI1 = 0.985
rIx = 1.042.
a 6.8196(3)
c 5.9742(3)
V, X = 277.842
100 x.
(Ndo.9oYbo. 1o)PO4 a 6.7221(6)
rvlI1 = 1.097
rIx = 1.151.
Vc M = 289.816
Wo.75~o.zW4 rvlI1 = 1.078
rIx = 1.133.
Vc M = 289.429
(Ndoso~osoF’O4
rvlI1 = 1.047
rIx = 1.102.
Vc M = 289.354
Wo.z~o.75)PO4 rvlI1 = 1.016
rIx = 1.072.
Vc M = 289.138
b 6.9339(6)
c 6.4007(5)
b 103.728(6)
a 6.7189(4)
b 6.9304(4)
c 6.3987(3)
b 103.738(4)
a 6.7187(5)
b 6.9280(5)
c 6.3995(4)
b 103.740(6)
a 6.7207(12)
b 6.9211(11)
c 6.3999( 10)
b 103.766( 17)
a 6.8361(5)
c 5.9881(6)
Vc X = 279.837
a 6.8356(4)
c 5.9867(4)
V, X = 279.73 1
a 6.8345(4)
c 5.9871(4)
V, X = 279.660
100 M.
77.7(0.6) M,
22.3(0.3) X.
47.7(0.5) M,
52.3(0.4) X.
21.4(0.5) M,
78.6(0.7) X.
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Table 4. Unit cell parameters of monazite and xenotime phases in (La.6Yv4)P04 heated to
temperatures of 12OO”C, 1500°C and 159O”C, with esd’s in parenthesis.
Compound Monazite phase cell Xenotime phase cell Relative proportion
parameters (A) parameters (A) (wt.%)
(La.6Y.4) 1200°C
40 hours
Vc M = 299.869
(La.gY.4) 1500°C
40 hours
Vc M = 294.132
(La.gY.4) 1500°C
275 hours
Vc M = 292.633
(La.gY.4) 1590°C
330 hours
Vc M = 291.598
NdP04
Vc M = 292.198
a 6.7902(4)
b 7.0158(5)
c 6.4729(4)
b 103.477(4)
a 6.7476(8)
b 6.9652(7)
c 6.4399(7)
b 103.637(7)
a 6.7365(4)
b 6.9523(5)
c 6.4305(4)
b 103.672(4)
a 6.8881(5)
c 6.0246(5)
V, X = 285.843
a 6.8893( 10)
c 6.0234(20)
Vc X = 285.885
a 6.8919(20)
c 6.0176(28)
Vc X = 285.826
73.4 (0.6) M,
26.6 (0.4) X.
89.3 (1.0) M,
10.7 (0.5) x.
94.9 (0.8) M,
5.1 (0.2) x.
a 6.7273(6)
b 6.9445(6)
c 6.4244(5)
b 103.697(5)
a 6.7428( 1)
b 6.9574( 1)
c 6.4103( 1)
b 103.675(l)
100 M.
100 M.
vi/ High temperature xenotime phase alteration
When samples of (Nd.35Ys65)P04 and (Sm.gY.g)P04 were heated to temperatures of 1500°C
and 1590°C powder XRD peaks in addition to those expected from the monazite and xenotime phases
were observed. Contrary to temperature dependent behaviour of (Nd.5Yb.g)PO4 and (La.gY.q)P04
compounds, the proportion of the monazite phase decreases with increasing temperature. The
compound (Nd_35Y_65)P04 changes from 42 wt % monazite at 1200°C to, what we believe is a single,
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distorted xenotime phase, at 1590°C. Preliminary SEM (in combination with EDS) work suggests that
there is only one phase of approximate formula (Nd.35Y.65)P04 within the sample heated to 1590°C.
All powder X-ray lines expected from the zircon structure type xenotime phase of rA{ REE3+}
slightly larger than the r of Tb3+ are present. The additional lines match closely reflections of the type
(h,k,l) and (h,h,l) with the restrictions of h+k+l = 2n and 2h+l = 4n respectively, removed. This
corresponds to a loss of I - centering and of the d - glide from the original space group : 14l/amd. The
single 4 - fold REE site within the zircon structure type could be lowered in symmetry to two 2 - fold
special positions which discriminate between the two different sized REEs.
We attempted to determine the structure of the ordered phase by Rietveld analysis of the
powder XRD pattern, by assuming that the structure ordered continuously as the REEs diffused to give
a preponderance of Nd in one of the two special positions. If this REE site discrimination process does
occur then the disordered structure should be able to be refined in the space group of the ordered phase.
Structural refinement in a number of suitably chosen space groups having either orthorhombic or
monoclinic unit cells was attempted with limited success. Refinement of REE positions took place
satisfactorily, but attempts to identify oxygen positions resulted in an unstable refinement. There still
exists a slight mismatch between the calculated and observed peak positions, suggesting that the space
group is actually triclinic. Single crystal X-ray diffraction or perhaps convergent beam electron
diffraction seem to be the only way to determine the space group of the ordered phase.
CONCLUSIONS
The Rietveld method has proved useful in the determination of the unit cell parameters of
monazite and xenotime phase mixtures. The degree of HREE substitution in monazite phases depends
on the difference in size of the two REEs and upon their rA. Large size differences between the two
REEs leads to only minor HREE substitution. Where the rA{REE3+} of the REEs is significantly
larger than the r{ REE3+} of the border element Gd, an increase in HREE substitution in LREEP04 is
seen with increasing temperature. The level of LREE substitution in xenotime phase HREEP04 also
depends on REE size difference and the rA{ REE3+}. There is however no increase in LREE
substitution with increasing temperature.
A lower symmetry, REE ordered phase, closely related to the zircon structure, forms in
compounds with REEs similar in size and with an rA{REE3+} slightly less than the r{ REE3+} of
Gd3+.
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Milligan, W. O., Mullica, D. F., Beall, G. W. and Boatner, L. A. (1983) C39,23.
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Mullica, D. F., Grossie, D. A. and Boatner, L. A. (1985) J. Solid State Chem. 58, 71.
Ni, Y., Hughes, J. M. and Mariano, A. N. (1995) Amer. Miner. 80,21.
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