J. CHEM. SOC. DALTON TRANS. 1982 1691

Part 2.t Defect Centres inEnergy Transfer in Actinyl Compounds.Na[U02(OOCCHah]

By Robert G. Denning: Charles N. Ironside. Timothy R. Snellgrove. and Paul J. Stone. InorganicChemistry Laboratory. South Parks Road. Oxford OX13QR

Time-resolved fluorescence measurements and excitation spectroscopy of single crystals of Na[U02(OOCCH3hJat 4.2 Kshow the presence of two types of perturbed centre whose relativeconcentrations appear to be independentof the method of preparation. A model is proposed in which the sodium ions are partiallydisordered.

IN a recent comprehensive study of the electronic spectraof the uranyl ion 1 the low temperature single crystal ab-sorption spectra, magnetic circular dichroism (m.c.d.), andnatural circular dichroism (c.d.) of Na[U02(00CCH3hJwere used to identify the number of electronic excitedstates and, in some cases, their symmetries. Theseresults and others 2 provided the basis for an interpre-tation of the electronic structure of the U022+ ion.3 Atroublesome feature of the spectroscopy of sodiumuranyl acetate is the observation, in the absorptionspectrum, of a number of components within 30 cm-1 ofthe electronic origins which cannot be interpreted asphonon structure. We had assumed 1that the consistentrelative intensity of these satellite features, regardless ofthe method of crystal growth, indicated that they arosefrom factor group splittings. In emission five shallowtraps have been observed 4 although it is not clear thatany of these are of sufficient abundance to be responsiblefor the satellite absorption. In this paper we show, bymeans of time-resolved emission and excitation spectro-scopy, that the satellite absorption is due to the samecentre which leads to one of the trap emission bands.However there are also centres which can be describedas anti-traps in the sense that their electronic energies lieabove those of the bulk. We conclude that the omissionof the satellite absorption from our previous analysis 1 isjustified and that the number of electronic excited statesderived from that analysis does not need to be modified.

EXPERIMENTAL

Large single crystals (4 x 4 x 4 mm) of Na[U02-(OOCCHaJaJ, usually taking the form of truncated octa-hedra, were grown either by slow cooling of saturated so]u-tions in a Dewar flask from ca. 70 °C or by diffusion of asolution of uranyl nitrate into gels formed by the neutralis-ation of sodium metasilicate with acetic acid. The crystalswere of a high degree of optical perfection.

The emission spectra were excited by a tunable. nitrogen-pumped dye laser (Molectron type DL200) operating at20 Hz with a bandwidth of 0.5 cm-l and a pulse length ofca. 5 ns. The emission was analysed by a Spex 1404 doublemonochromator. The time evolution of the luminescencewas recorded by a Biomation 8 100 transient digitiserhaving a minimum digitisation interval of 10 ns and a2048 8-bit word memory. The results from successivelaser pulses were averaged in real time by transfer of thedata to a Research Machines Ltd. 380Z microcomputer via

t Part 1 is ref. 7.

a fast parallel interface. G The excitation spectra of variousemission features were obtained by integrating the totalluminescence at the appropriate wavelength after each laserpulse and normalising the result with respect to the ampli-tude of the excitation pulse as recorded from a photodiodevia the second channel of the digitiser.

The chemical purity of the samples was determined byinfrared spectroscopy, chemical analysis, and thermogravi-metric analysis. Crystals grown from D20 solutions haveidentical Lr. spectra to those grown from H20, so that bothwater and co (OH) groups cannot be present in the material.Similarly the thermogravimetric analysis excludes thepresence of water in the crystal; the decomposition occursby loss of acetate to generate Na2U207.6 Analysis ofNa[U02(OOCCHa)a]: Found: C, 15.35; H. 1.75; Na.4.80;U, 50.70. C6HsNaOsU requires C. 15.30; H, 1.90; Na,4.90; U.50.65%.

RESULTS

The time-resolved emission spectrum after excitation at4535.6 A. Figure I, consists of progressions in the UOasymmetric stretching frequency. It enables three types ofemitting centre, with different time dependence. to beidentified. For example the two features near 4 900 A,which repeat near 5 120 A, decay with a characteristic timescale of 10 fLswhereas the relative intensity of the two fea-tures near 4 935 Achanges on a much longer time-scale.

The resolution of the emission bands allows the excitationspectrum of each of the three centres to be recorded inde-pendently (Figure 2). Each spectrum begins with a sharpfeature, or features, which we take as either the electronicorigin or two origins in close proximity. The centres arelabelled (A), (B). and (C) in order of increasing energy of thefirst excited states.

The excitation spectrum of the (B) centre agrees well inmost respects with the absorption spectrum of the bulkmaterial (rei. I, Figure 8). The energy of the first excitedstate is at 21 135 cm-l corresponding to the label Aa in thenomenclature of ref. 1. The origin of the (A) centre absorp-tion occurs at 21 104.1 cm-l and is identical with the satelliteabsorption labelled Al in ref. 1. Centre (C) has origins near21 240 cm-l and constitutes what we have termed an anti-trap.7

Because (A) and (C) are minority absorbers it cannot beassumed that their excitation spectra give a completerepresentation of their absorption spectra. The samples arenot optically thin with respect to the absorption of the bulk(B) so that the mean flux incident on the minority centres isa function of the degree of absorption of (B) at the appro-priate wavelength. Strong absorption by (B) will preventthe excitation of (A) and (C). Therefore. in comparing the

1692

spectra, only those features which are present, as opposed tothose that are absent, are significant.

With this proviso it is evident that the main features ofthe spectra are very comparable. For example the spec-trum of (A), although not completely separated from that of(B), clearly shows similar vibronic structure on its first

J. CHEM. SOC. DALTON TRANS. 198

resolution of the degeneracy by the perturbation of thdefect.

More information as to the nature of the three types (centre can be obtained from an analysis of the. time-resolve-emission spectra under high resolution. Figure 3 showsportion of the emission spectrum, excited at the wavelengt

5 180 5 020

Wavelength ( A)FIGURE 1 Time-resolved luminescence spectra of Na[UO.{OOCCH.).] at 5 K

5500 5340

electronic origin to that occurring on the equivalent transi-tion in the bulk; the whole spectrum is shifted by 31.5 cm-1to lower energy.

Near 4 560 A the excitation spectrum of (A) is effectivelycut off by the absorption of the bulk, but the progressionsbased on some of the features can be observed at wave-lengths shorter than those illustrated in Figure 2.

The excitation spectrum of the (C) centres shows a strongresemblance to that of (A) despite an overall shift of 130cm-l to higher energy. Progressions based on many of thefeatures in Figure 2 can be traced in the excitation spectraup to 4 250 A. Table 1 summarises the observed energiesof the electronic states associated with the three centres andthe principal vibrational features built upon them. Thenomenclature 1 in which the multiplicity of the componentsin the progressions has been analysed takes account of theinteraction of the modes labelled Vsym.and Vn,which havethe same symmetry. Features associated with the (C)centre are doubled and are reported in Table 1 as (C) and(C'). The linewidth of the emission is such that it is notpossible to distinguish through the time dependence of theiremission whether (C) and (C') are distinct centres, orseparate levels of a single centre. Both of the first twoelectronic excited states in Na[U02(OOCCH3bJ are de-generate 1 so that it is possible that (C) and (C') reflect the

4860 4700

at which the (C) centres absorb, integrated over a period 0200 !Ls after the excitation pulse. Comparison with th-spectra integrated over 50 (Josand 2 ms, which emphasiz-

TABLE 1

* For nomenclature see ref. 1.

the features of the (C) and (A) centres, respectively provides a straightforward assignment of the bands of thindividual centres. The emission of (C) is split into tWIcomponents. In Figure 3 the (B) emission is self-absorbedThe emission at wavelengths longer than those shown i:Figure 3 resolves satellites to the (B) features, labelled (Band (B'). which have slightly different progression fre

,....Principal coupled modes *Site Origin Energy/cm-l VBym Vasym VD

(A)nI

21 103.4 732 701.9 683.321 903.3 - - 680.3

675.2(B) nI

21 135.0 729 702.8 681.421 915.3

(C) nI21 325.622047.6 734.2 - 691.3

725 - 678(C') nI

21245.7 - - -22061.6 739.6 - 691.3

725 - 678.4

I. CHEM. SOC. DALTON TRANS. 1982 1693

Centre (A)

4 570 4 630

Wavelength (A)FIGURE2 Luminescence excitation spectra of centres (A), (B), and (C)at 5 K

4450 4510

quencies to those of the bulk. They probably representmolecules removed by a lattice spacing from the defect sites.Table 2 summarises the observed sideband frequencies inemission and their progression frequencies.

DISCUSSION

The data in Table 2 illustrate that there are no majordifferences in the emission spectra of the three types ofcentre. Where they are observed the same vibronicfeatures can be identified in each case. In particular theprogression frequencies vary very little in associationwith the various sidebands, with the exception of thecombination with the asymmetric U02 strecthing modeVasym,where appreciable anharmonic coupling is observed;the same is true in other cases.2 Compared with thevariation in Vsym on anyone centre there are appreciabledifferences in the frequencies on the (A) and (C) centres(848.7 and 847.6 cm-I, respectively) as compared to thevalue for the bulk material (854.0 cm-I). The perturbedsites apparently experience a small weakening of the UObond.

Centre (B)

Centre (C)

4 690

Some conclusions may be drawn from the relativeintensities in the emission spectra illustrated in Figure 4.The principal intensity lies in features associated with thesymmetric and asymmetric U02 stretching frequencies,Vsym and Vasymrespectively. The intensity of the bandsinvolving only the symmetric stretching mode may beexpected to reflect the small degree of electric dipoleintensity static ally introduced into an essentially parityforbidden transition, whereas those involving the asym-metric mode have an intensity related to the dynamicperturbation of this mode. It is unlikely that thedynamic mechanism is strongly perturbed in the defectcentres so that the relative intensity observed for the twotypes of mode is a measure of the static intensity intro-duced into the pure electronic transition at the defectsite. A comparison of these relative intensities for the(A) and (C) centres near 4 900 A indicates an increase inthe static intensity at the (A) centre. The comparisonwith the equivalent features of the bulk emission isdifficult at this wavelength because of the presence ofsatellite structure due to the (B') and (B") centres.

1694

(C)

(8)

(A)(8)

(8)

J. CHEM. SOC. DALTON TRANS. 1982

(8)

(A)

(8) (8)

(8

(A)

4956

FIGURE 3

4 828 4 892

Wavelength (A)Luminescence spectrum at 5 K, integrated over 200 p.s, excitation wavelength 4535.6 A

4700

Originfcm-l

{21 102.3

-Veym

- 2v,ym- 3v,ym

{21 135.0

-V,ym- 2v,ym- 3v,ym

{_21 135.0-v- 2v,ym- 3v,ym

{-21 135.0

-Vsym21 235.6

}. .

21 245.8 -V'ym- 2v,ym. For nomenclature see re£. 1.

frequency. .

Site

(A)

(B)

(B')

(Bn)

(C')(cn)

4764

TABLE 2

V,ym

o848.7844.5842.5o853.6852.3848.5o849.5850.8847.4o855.7o847.6846.3

124.8854.6850.4

163.4

922.3847.7844.8

931.5852.5927.4840.8838.4

h Anomalous intervals due to Fermi resonance. . Intervals calculated from mean origin

Nevertheless it is clear from the spectra near 5 150Athat,while both modes have similar intensities when theyappear on the (B) cenfre, the statiC intensity is againrelatively enhanced on the (A) centre. The staticintensity on the (A) centre is approximately twice that inthe bulk. We may then use the relative intensity of the(A) and (B) origins in absorption (1 : 10) to estimate thatthe relative abundance of the (A) centre is approxi-mately 5%.

A careful assessment of the chemical and spectro-scopic analyses of the material shows no evidence ofchemical impurities with this abundance. It musttherefore be assumed that the perturbed sites are

to be associated with disorder in a chemically purematerial.

The crystal structure of Na[U02(OOCCHa)a]is known.8The material is cubic, space group P213, and is enantio-meric. There are four molecules per unit cell; eachuranium atom lying at a Ca site. The natural opticalactivity as observed in the c.d. spectrum 1is attributableto the helical canting of the bidentate acetate chelaterings out of the equatorial plane. This distortion isresponsible for the non-zero electric dipole transitionmoment in the first electronic origin, perpendicular tothe Ca axis. The natural c.d. of site (A) is so similar tothat of the bulk sites 1 that the local stereochemistry of

Pdncipal vibrational and progression intervals.Vr Vi> Vasym

133.9 - 164.7 240 267.2 606.2 - 921.2848.8 . - 849.7 848.8 - 842.3844.0 - 843.7 845.3 - 840.0842.0 - - - -

134.7 136.9 163.3. 237.8 269.4 606.9 676.2 926.4.853.6 854.6 854.7 853.1 856.3 b 854.5 850.1852.3 851.9 852.3 854.3 850.9 h 853.1 846.9

. 849.7 849.1 849.3 848.2

J. CHEM. SOC. DALTON TRANS. 1982

the chelate rings does not seem to be strongly affected.Because the vibrational data obtained from the emissionspectrum also show no sign of the disruption of the im-mediate co-ordination sphere of the uranyl ion we assumethat the complex anion [U02(00CCHa)a]- retains itsstructural integrity in the defect sites. If this is truethe only viable source of structural disorder lies in thedisplacement of the sodium ions.

(B)(a)

(C) (C)

4860 4892 4924

(b )

1695

of the cell to the top left corner. On this axis a sodiumion occurs at the top left and a uranyl group just belowthe centre of the cell. A large empty volume occurs atthe bottom right. Figure 5(b) shows the view alongthe {HI} direction. At the centre of this view thecentral uranyl group is seen end on and the channel-likeform of the empty region is clear. There are threeadditional vacancies related by symmetry to this volume.

(A)

4956 4988(A)

(A)

5164

Wavelength (A )FIGURE 4 Luminescence spectrum at 5 K; (a) integrated over 200 ILs, illustrating relative intensities in the (A) and (C) emission;

(b) integrated over 2 ms, emphasizing relative intensities in the (A) emission; and (c) integrated over 200 ILs illustrating therelative intensities of the (A) and (B) features

4860 4892

(c )

5 020 5068 5116

Normally the sodium ions occupy sites of almostoctahedral symmetry, the ligands being carboxylateoxygen atoms. The sodium ions act as bridges betweenadjacent carboxylate chelate rings within the equatorialplane of the uranyl ions. Each sodium thereby linksthree complex anions. Within the unit cell four sodiumions are tetrahedrally disposed, lying on the three-foldaxes. With respect to anyone uranyl ion three of thesesodium ions surround it slightly below the equatorialplane while the fourth lies well above the plane along thethree-fold axis coincident with the O-U-O direction.

This situation can be discerned in Figure 5(a) whichshows a view of the unit cell in the {HO}direction. Theatoms have been given their van der Waals radii. Fromthis viewpoint a three-fold axis links the bottom right

5 212

Figures 5(c) and 5(d)show these volumes occupied byspheres with a diameter of 4.00 A, chosen so that they donot overlap with the surrounding atoms. There are noother voids of appreciable size within the structure.

The van der Waals diameter of sodium is 2.9 Aso thatit is easily accommodated in the vacancies. We there-fore propose that the disorder in the structure arises fromthe partial occupancy of these vacant pockets by sodiumions. Only a small change in the cation-cation repulsionoccurs if the Nal atom in Figure 5(a) is displaced alongthe {HI} direction into the volume labelled Val (Varepre-sents the vacancy occurring in the unit cell) [in Figure5(c)] in the adjoining unit cell. In this location it liesclose to the uranyl oxygen atom 01 in Figures 5(a) and5(b) and also to the three carboxylate oxygen atoms ofthe

4924 (B) 4956 4988.(B)

(A)(A)

11ft \A(C) (C)

1696

original site, OIl. The vacancy has a cylindrical channel-like shape and is surrounded by a number of methylhydrogen atoms and a further three carboxylate oxygenatoms, 01II [Figure 5(b)J. Assumingsome deformationof the surrounding acetate groups this appears to be anelectrostatically viable location for the sodium ion.

(a)

(c)

FIGURE 5

J. CHEM. SOC. DALTON TRANS. 1982

[type(ii)) which is perturbed by a sodium ion lying closeto and co-axial with the uranyl oxygen atom, 01.

For type (i) ions it may be anticipated that the equa-torial field of the ligands increases as a result of theremoval of the competition of the sodium ion for thecharge on the acetate oxygen atoms. This perturbation

b

b

(b)

o

b

b

(d)

o

(a) Unit cell viewed in the {HO} direction; (b) unit cell viewed in the {HI} direction; (c) same as (a) showing vacanciesof diameter 4.0 A; (d) same as (b) showing vacancies of diameter 4.0 A

This sodium displacement creates two types of defect-ive uranyl centre simultaneously. There are threeuranyl ions [type (i)] from whose equatorial plane thesodium ion has been withdrawn and a single uranyl ion

should raise the average energy of the metal! orbitals inthe equatorial plane and raise the energy of the electronictransition (for the nature of the transition see re£.3). Wetherefore associate the type (i) ion with the (C) spectro-

J. CHEM. SOC. DALTON TRANS. 1982

scopic centres. Consideration of the intensity mechan-ism for the first excited state 3 leads to the conclusionthat a perturbation applied in the equatorial plane is lesseffective in providing electric dipole intensity than out-of-plane perturbations. It follows that no major changein static electric dipole intensity should be expected inthe (C) centre.

Type (ii) ions can be expected to suffer an elongationof the uranyl-oxygen bond. Where cations competefor this oxygen atom lengthening of the bond is wellestablished.9 Furthermore the lengthening is ac-companied by (a) a decrease in the U-o stretchingfrequency, and (b) a lowering in the energy of the firstelectronic excited state.1O We therefore believe that the(A) centres should be identified as type (ii) ions. Be-cause the perturbation lies perpendicular to the equa-torial plane we may expect an appreciable increase in thestatic electric dipole intensity of the first excited state, asobserved in the (A) centres. Finally it should be pointedout that type (i) and (ii) ions should always be present inthe ratio 3 : 1 under this model. It follows that the (C)and (A) centres should always be present in this ratio.As far as we can judge from the intensity of their lumin-escence this is true of the samples which we have studieddespite the fact that the crystals were grown by varioustechniques.

The discussion presented here supports the presence ofan appreciable concentration, ca. 5%, of defects associ-

1697

ated with displaced sodium ions. There is quite rapidenergy transfer between the bulk and the traps. Theseobservations account for the satellite features observedin absorption and imply that the number of electronicexcited states previously assumed 1 is correct.

Wc acknowledge the support of the S.RC. and A.E.RE.Harwell (for P. J. S.), and the S.RC. for the purchase ofequipment. We are indebted to J. O. W. Norris andJ. R. G. Thome for the development of computer programsused in the data analysis, and to K. Davies for the pro-grams used to display the contents of the unit cell.

[2/118 Received,21st January, 1982]

REFERENCES

1 R G. Denning, D. N. P. Foster, T. R. Snellgrove, and D. R.Woodwark, Mol. Phys., 1979, 37, 1089.

2 R. G. Denning, T. R. Snellgrove, and D. R. Woodwark, Mol.Phys., 1976, 32, 419.

3 R. G.Dcnning,T. R. Snellgrove,and D. R. Woodwark,Mol.Phys., 1979, 37, 1109.

4 A. P. Abramovand I. K. Razumova,Opt.SPektrosk.,1975,38,565.

5 R. G. Denning and C. N. Ironsidc, J. Phys. E., 1981, 14,468.

8 T. C. Tso, personal communication.7 R. G. Denning, C. N. Ironside, J. R. G. Thorne, and D. R.

Woodwark, Mol. Phys. 1981, 44, 209.B W. H. Zachariasen and H. A. Plettingcr, Acta Crystallogr.,

1959, 12, 526.9 W. H. Zachariasen, Acta Crystallogl'., 1954, 7, 795.

10 W. T. Carnall, S. J. Neufeldt, and A. Walker, [norg. ChCIII.,1965, 4, 1808; T. P. Softley, Part 11 thesis, Oxford, 1981.