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Raman Spectra of the Rare EarthOrthophosphatestG. M. Begun, G.W. Beall, L. A. Boatner an d W. J. GregorChemistry and Solid State Divisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, US A

Raman spectra of anhydrous crystalline samp les of all of the lanthanide orthophosphates (with the exception ofP m P 0 4 ) have been ob tained. These data were interpreted in a systematic manner based on the known structuresof these com pounds. Assignments and correlations have been made fo r many of the observed bands.

INTRODUCTIONOrthophosphates formed by elements in th e first half ofthe lanthanide transition series are structural analogsof the monoclinic mineral monazite. This mineral ischaracterized by an almost unique combination ofproperties that make its synthetic forms attractive aspossible hosts for the long-term storage of radioactiveactinide Accordingly, a series of investigationsof the chemical and physical properties of the lanthanide(and related) orthophosphates is currently in progress.Single crystals of all of the lanthanide orthophosphates(except PmP0 ,) doped with small amounts of Gd3+were prepared for the initial purpose of performing aseries of electron paramagnetic resonance experiments.The availability of these samples made it possible tocarry out a study of the Raman spectra for essentially theentire series of rare-earth orthophosphates. This studywas directed toward the following objectives. First,baseline spectra were to be established that could beused as a basis of comparison with subsequentlanthanide or thophosphate crystals that would be dopedwith @-active actinides and, consequently, would besubjected to heavy-particle radiation damage. Second, aclassification and correlation of the Raman spectra ofthe entire group of rare earth orthophosphates wasdesired. Yttrium orthophosphate was included in thepresent investigation because its properties are knownto correlate well with those of the orthophosphates ofthe second half of the rare-earth series (i.e. Tb P0 4through LuP 04) and also because YP04 s the analog ofthe natural mineral xenotime. In addition, a consider-able literature exists on the properties of YP 04 hat canbe used for the purposes of verification and comparisonwith the lanthanide compounds.The lanthanide orthophosphates can be convenientlydivided into two groups on the basis of the prevalentcrystal structural form. The first group has the mono-clinic 'monazite' structure and consists of LaP04through G dP04. The second group has the zircon struc-ture and consists of T bP04 hrough LuP04. Both YP 04and S c P 0 4 also crystallize with the zircon structure andcan be grouped with the orthophosphates formed by

f A portion of this material was presented at the VIIth Internationa lConference on Raman Spectroscopy at Ottawa, Canada, August1980. See Proceedings VIIth International Conference on RamanSpectroscopy, ed. by W . F. Murphy, p. 88. North Holland PublishingCo., New York (1980).

elements in the last half of the lanthanide series.Promethium is not a naturally occurring element, and itwas not practical to investigate the orthophosphate ofthis unstable nuclide.The Raman spectrum of crystalline Y P 04 has beenreported by Ri~ hr na n; ~nd Lazarev et aIa4 ave presen-ted the Raman spectra of Y P0 4, HoP 04 and ErPO,. Inthe latter work, single crystal studies. were performedwhich enabled assignments of most of the frequencies tobe made and attempts were also made to find systemati:variations with the atomic number. Yurchenko et al.have published Raman spectra of Y, Tb, Dy, Ho, Tm,Yb and Lu orthophosphates but many of the spectra arefragmentary and no significant attempt was made tosystematize the results. The Raman spectrum of D yP04has been published by Elliott et aL6 Reports of theRaman spectra of the Group I (i.e. lanthanum to gado-linium series) of rare earth or thophosphates were notfound in the literature.

The IR spectra of lanthanide orthophosphates havebeen fairly extensively studied. Infrared studies ofYP0 4, HoP0, and Er P 04 are reported in Ref. 4 citedabove, while the IR spectra of most of the Group I1compounds accompany the Raman spectra in Ref. 5 .The IR spectra of most of the Group I and Group I1orthophosphates were examined p y Hezel and Ross' inthe spectral region 400-4000 cm- .Their work is part ofa comprehensive study of 63 phosphates, sulfates andperchlorates. Portions of the IR spectra of La, Ce, Eu,Gd , Y and Y bP 04 have been reported by Tenisheva eta1.* Reflection IR spectra of Lu P04 and YP 04 havebeen reported by Armbruster,' and the IR absorptionspectrum of GdPO, is given by Petrov ef al.'"

EXPERIMENTAL~~~~ ~ ~

Crystals of the lanthanide and yttrium orthophosphateswere grown by dissolving and reacting lanthanide oxidesin molten lead pyrophosphate at high temperatures."The L nP04 crystals which formed on cooling wereseparated by dissolving the lead phosphate matrix inboiling concentrated nitric acid. Lanthanide oxides witha purity of 99.9% or better were used for all of thecrystal preparations. The samples were doped by adding0.1 wt% GdZ 03 elative to the lanthanide oxide. The

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G. M. BEGUN, G. W . BEALL, L. A. BOATNER AND W . J. GREGORgadolinium impurity did not produce any detectableeffects in the Raman spectra.Raman spectra were recorded with a Ramanor HG2Sspectrophotometer (Instruments SA). This instrumentemploys a double monochromator with curved holo-graphic gratings and photoelectric detection and pulsecounting electronics. The spectra were excited with the514.5, 488.0 or 457.9nm lines of a Spectra-Physicsmodel 164 argon ion laser. The samples were generallyin the form of small crystalline powders contained inmelting point tubes. Some orientation effects wereobserved for the larger zircon-structure crystals, andsome difficulties were encountered with the excitation ofelectronic bands, especially in the cases of Ce, Eu and Tborthophosphates. By observing spectra with severaldifferent exciting lines, it was generally possible to dis-tinguish the Raman spectra from the interfering elec-tronic transitions. A few extraneous lines that wereobserved in some of the spectra in the 1100 cm-' regionmay be due to the inclusion of small amounts of meta- orpyrophosphates in some of the samples.Polarization studies were made on single crystals ofLaP04and PrP 04 hat were oriented, using Laue X-rayback reflection techniques, with the axis of symmetryperpendicular to both the incident and scattered light.Significant polarization effects were observed. It was notalways possible, however, to distinguish between the A ,and B , modes due to overlapping or unresolvedcombinations of the spectral bands.

t+.-u)a,C+I

1La PO4Excitation 488.0 nml I

I C ~ P O ~1 Excitot ion 514.5 nm

Pr PO4Exci ta t ion 514.5nrn: I

S m P o 4 I 1

A Wavenumbers (ern-')Figure 1. Raman spectra of the Group I (La, Ce, Pr, Nd, Sm, Euand Gd) orthophosphates.

274 JOURNAL OF RAMAN SPECTROSCOPY, VOL. 11, NO. 4, 1981

t2.cC)CtH

Figure2. Raman spectra of the Group It (Y ,Tb, y, Ho, r, Tm, Yband Lu) orthophosphates. *=plasma lines.

RESULTS AND DISCUSSIONRaman spectra of the group I (La, Ce, Pr, Nd, Sm, Euand Gd) orthophosphates are shown in Fig. 1 . Figure 2presents the Raman spectra of the Group I1(Y, Tb, Dy,Ho, Er, Tm, Yb and Lu) compounds. Figures 3 and 4show the results of polarization studies on a single crystalof PrP04 . The frequencies of the observed Ramanbands, their intensities, and our vibrational assignmentsare listed in Tables 1and 2. A comparison of our results

2,tcc(I E100

A Wavenumbers (cm-')Figure 3.Single crystal polarized Raman spectra of PrP04.X ( Z X ) Vand X(ZZ)V.514.5 nm excitation.

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RAMAN SPECTRA OF THE RARE EARTH ORTHOPHOSPHATES

A Wavenumbers (ern-')Figure 4. Single crystal polarized Raman spectra of PrP04.X ( Y X ) Y and X ( YZ ) Y .514.5 nm excitation.

with the published data shows good agreement betweenour band frequencies for YP04 , Ho P0 4and ErP 04 andthose published in Ref. 4. The spectrum of DyP04shown in Fig. 2 is in excellent agreement with thatpresented by Elliott et aL 6 Although there is consider-able agreement, the Group I1 frequencies determinedhere do not coincide as well with the rather sketchy dataof Ref. 5 for the Y, Tb, Dy, Ho, Tm, Yb and Luorthophosphates.The present Raman observations may also becompared with the infrared observations of Hezel andRoss.' The free (P04)3p on has the four normal modesof vibration of a tetrahedral ion. These are: vl(Al),symmetric stretching; v3(F2),ntisymmetric stretching;

v*(E)and v4(F2), bending vibrations. The Group I1orthophosphates would be expected to have twofrequencies, both IR and Raman active, in the P - 0bond stretching region [the v3 region of ion].With the exception of 3 or 4 IR frequencies, very goodagreement is found with these data. In the o ther regionsno coincidences are expected between the Raman andIR data and this was found to be the case. The Group Iorthophosphates present much more complicated IRand Raman spectra. The IR data of Hezel and Ross7were used for comparison. The crystal site symmetry ofthe ion in this case indicates all of the frequen-cies should be both IR and Raman active. While acomparison of the IR and Raman data shows that thereare numerous agreements, some frequencies are clearlynot coincident. We attribute this lack of coincidences tofactor group splitting which would give rise to mutualexclusion between the IR and Raman frequencies. Thispoint may be illustrated by the frequency derived fromthe v 1 symmetric frequency of the ion where thepresent assignments are consistently 20 cm-' above theIR assignments of Ref. 7 .The Group I1 (Y, Tb, Dy, Ho, Er, Tm, Yb and Lu)anhydrous orthophosphates crystallize with the tetra-gonal zircon structure. The crystals belong to spacegroup0:; (14:lamd).'* There are two molecules in theprimitive cell. The Y ions and the groupsoccupy sites, while the oxygen atoms are located atsites with C, symmetry. The zircon structure and itsvibrational modes are treated quite thoroughly byDawson et ~ 1 . ' ~n a paper dealing with the vibrationalspectrum of ZrSi04. Factor group analysis gives the

Table 1. Experimental Raman frequencies (cm-') and relative intensities of the monoclinic,group I, lanthanide orthophosphates'No

25A24A23A22A20A19A18A17A16A15A14A12A11A1OA9A8A7A6A1A2A3A4A5A

La bgO(l.6)lOO(0.6)120(0.2)131 0.4)151 0.5)170(0.5)183(0.3)220(1.O)227(1.I)255(0.4)271 (1 O)394(0.9)414(1.5)465( 1.7)537(0.4)57 2(0.5)589(0.3)61 g(0.8)

967(10.0)991 (1.7)1025(0.4)1055(2.9)1065(0.4)1073(0.8)

Ceb88(1.5)lOO(1.0)118(0.2)129(0.7)152(0.5)172(0.6)188(0.1)21 g(1.4)227(1.l)254(0.4)270(0.9)396(1.3)414(1.8)467(2.6)536(2.5)571 (0.6)589(0.3)618(1.2)

970(10.0)99011.7)1024(0.4)1054(3.4)1070(1.1)

P P90(1.7)105(0.4)122(0.2)133(0.4)158(0.5)177(0.8)188(0.1)224(1.2)230(sh)251 (0.4)282(0.8)399(0.9)417(1.5)469(2.0)538(0.3)572(0.4)592(0.2)623(0.9)

974(10.0)994(1.I1028(0.4)1058(3.7)1075(1.0)

a All doped pi th 0.1 wt.% Gd.Excitation: 514.5 nrn, 488.0 n m ; sh =shoulder.

NdC89(8.2)106(2.3)123(1.4)133(3.1)155(1.5)164U.9)180(4.0)227(6.9)230(sh)263(2.3)286(3.4)399(3.8)420(5.4)470(5.7)539(0.8)574(1.7)595(0.8)625(3.0)

977(10.0)997(1.7)1032(0.7)1062(5.2)1079(1.9)

Smb88(1.3)107(0.5)123(0.4)132(0.6)156(0.3)170(0.5)185(0.6)2320.1)242(0.6)265(0.4)294(0.4)402(0.8)423(1.2)473(1.2)538(0.3)575(0.6)595(0.4)627(0.8)

982(10.0)999(0.7)1035(0.4)1065(2.2)1OSS(0.6)

Eu b87(1.8)108(0.5)124(0.5)132(0.8)158(0.3)175(0.5)189(0.7)233(1.9)244(0.9)265(0.8)310(0.7)403(1.2)423(0.7)471 (2.3)538(0.3)576(0.5)597(0.3)629(1.0)945(0.7)989(10.0)

1069(2.4)1082(0.3)1093(0.8)

Gdb87U.4)108(0.3)123(0.4)130(0.6)158(0.2)178(0.5)192(0.5)236(1.I247 (0.6)270(0.5)308(0.4)404(1.O)429(0.4)480(1.5)540(0.3)579(0.5)599(0.3)632(0.9)

987( 10.0)1004(0.5)1043(0.5)1072(2.2)1092(0.7)

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Table 2. Experimental Raman frequencies (can-') and relative intensities of the xenotine type, tetragonal, group 11, lanthanideorthophosphates"No. Yb Tb b DyC Hob Erd T d YbC L"b Assignment

-96(0.2) 98(0.05)132(0.5) 138(0.3) &,(TI186(0.3) 191(0.3) Bi,(T)138(0.5) 144(0.4) E, (T )

158 267(0.3) 267(0.5) 266(0.3) 268(0.5) E , ( T )14 8 299(3.0) 293(3.0) 295u.9) 297(10.0) 300(3.3) 303(1.6) 302(0.5) 308(2.3) E, (R )13 8 332(0.3) 331 (0.7) 331 0.7) 336U.6) 330(0.2) 330(0.1) 330(1.O) 334(0.6) B2g10B 484(0.9) 484(1.6) 485(1.1) 486(4.6) 477(2.0) 488(1.5) 491(1.1) 493(1.3) Al,8 8 581 (0.8) 576(0.6) 578(0.4) 578U.4) 580(0.6) 580(0.6) 582(0.5) 587(0.7) 46 6 660(0 .1) 649(0.8) 654(0.5) 656(0.5) 659(0.1) 660(0.05) 663(0.5) 670(0.3) 61918 1001 10.0) 995(9.6) 998(10.0) lOOl(3.5) 1004(10.0) 1006(10.0) 1009(10.0) 101 (10.0) Al,2 8 1027(1.9) 1014(2.7) lOlg(1 .2) 1021(0.7) 1024(1.9) 1027(1.6) 1030(1.3) 1032(1.8) 41048(0.5) 1043(sh) 1048(0.5) -3 8 1058(2.0) 1049(10.0) 1054(6.4) 1055(0.4) 1061(1.4) 1064(0.4) 1068(8.1) 1069(3.2) 8 1

{ c;;::::;28 121(0.05) 130(1.1) 132(0.4) 132(1.8)21 8 157(0.7) 141(1.4) 141 0.6) 141 1.5)18 8 185(0.2) 183(0.3) 185(0.1) 184(0.7) 185(0.3) 185(0.2)210(0.1) -

a Al l doped with 0.1 wt.% Gd.Excitation: 514.5 nm, '488.0 nm, 457.9 nm; sh =shoulder.

representation of the optical vibrations asrT = 2A1, +Azn+4B1, +Bz, +5E, +A, ,+3A2, + B1, +2Bzu+4E,.

This leads to Raman active frequencies as follows:rEXT=2BIg+3E,: rINT=2A1g+2B1g+Bzg 2E,

for a total of twelve Raman vibrational bands. Lazarev etd4 ave made single cI ystal polarization studies ofY P 0 4 , HoP0 4 and ErP04 and assigned the observedRaman bands accordingly. We have followed theirassignments and our data and assignments are tabulatedin Table 2 for the Group I1 orthophosphates.

Table 3 lists the correlation scheme for the phosphatestretching and bending modes between the isolated

Table 3. (Po4)'- group symmetryXenotime

crystalFree ion Site symmetry symmetry

Td D2d 0 6 ,

tetrahedral (P04j3 - group, the (P0,j3- ion located at aDZd ite, and theD4,,actor group splitting in the crystal.It can be seeq from the data of Table 2 that a simple sitegroup analysis is not adequate to explain the spectra butthat the factor group analysis fits quite well.The Group I (La, Ce, Pr, Nd, Sm, Eu and Gd)orthophosphates cr stallize with the monoclinicmonazite [space group (P21/n)].12Thereare four molecules per unit cell. All atoms are located atgeneral positions (Cl).Site group analysis predicts thatall of t he modes should be both IR and Raman active ofClass A. This, however, does not agree with the data anda factor group analysis leads to the optical vibrationsrT= 18A, + 18B, + 17A, + 16Bu. The Raman activefrequencies are then: TINT= 9A, +9B,; rEXT9A, + 9 B , for a total of 36 Raman active modes. Forthis group of lanthanide orthophosphates, at least 23frequencies were observed. Since the groups arelocated at sites of C1 symmetry, site group analysiswould yield 15 A modes, both IR and Raman active,whik factor group analysis yields 36 Raman and 33 IRactive frequencies as stated above. Clearly there isconsiderable factor group splitting since a com arison ofour data with the IR data of Hezel and Ross shows anapparent coincidence between the I R and Raman bandsin only 5 out of 13 observed IR bands. An assignment ofthe observed bands to A, or B, symmetry has beenmade in Table 1. This assignment is not unequivocalsince some of the bands a re clearly not split or areunresolved. Where both A, and B, symmetry are listed,either a distinction was not possible or both componentsare present.In order to classify the data systematically it is inter-esting to plot the variation of the observed frequenciesof the various vibrations versus the atomic number orthe crystal radii of the lanthanide ion. Figures 5 and 6 areplots of the symmetrical stretching frequencies of the(P04)3-group (no, 1A and lB, respectively in Tables 1and 2) versus atomic number and versus crystal radii.The crystal radii were taken from Ref. 17. The datapoint for yttrium does not fall on the curve in the atomicnumber plot but falls nicely on the line near Ho whenthe crystal radii are used. The symmetric stretching

P

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RAMAN SPECTRA OF THE RARE EARTH ORTHOPHOSPHATESI I l l I

I00810041000- 996-

'5 9 9 2 -1988

0 )p 98 4r=98097697296 8

----

---~

-

-9 6 4 l I ' I l l I I I3 9 ' 57 59 61 63 65 67 69 71

A t o m i c numberFigure 5. Symmetricorthophosphates v. atomic number.stretching frequencies for rare earth

frequency of the (P04)3- ion in the crystals increaseslinearly with the atomic number or crystal radius of thelanthanide ion from 967 cm-' in LaP 04 to 1011 cm-' inLuP04. In contrast, the symmetric stretching frequencyof the ion in the free state has a value14 of938 cm-'.Two series of peaks are found that are derived fromthe v3 antisymmetric stretching mode of the (P04)3 - ionat 1017 em-'. These are the 2A-2B and 3A-3B seriesshown in Fig. 7. In addition to these graphs the 4A and

1015

1010

1005

I000-' 995a_

ZI

g 9900 )0

985

980

975

970

965

I I I I I I

N d/7

Lo32 128 124 120 116 112I I 1 I

C ryst a l rodi i of M+3 ( p m )Figure6. Symmetric (P04)3-stretching frequenciesfor rare earthorthophosphates v. crystal radii.

::::II055

J 1025

1015

Is: 020 //A010 1 -1

1005I000

99098 5 I 57 59 61 63 65 67 69 71A t o m i c number

Figure 7. Antisymmetricearth orthophosphates v. atomic number.stretching frequencies for rare

5A series (see Table 2) must be due to a splitting of theantisymmetric stretching frequency of the ion.All the frequencies increase regularly with increasingatomic number. bending frequencies thefrequencies 6A and 6B, 8A and 8B, and 10A and 10Bare plotted in Figs 8 and 9. In a manner similar tothe stretching modes, the frequencies of these bandsincrease regularly as one progresses to the highermolecular weight orthophosphates. In the region of thelattice vibrations, correlations are more difficult, and18A and B and 22A and B have been plotted(Fig. 9) as representatives. In this region a slight ten-dency for the vibrational frequencies to increase withatomic weight is still found but is not nearly so markedas in the case of the higher frequencies.Efforts to find systematic trends were made byLazarev et d4 ho concluded on the basis of their datathat most of the frequencies decreased with increasingatomic number. These authors calculated forceconstants to support this contention. The results of thepresent investigations, however, clearly show that thedominant trend is in the opposite direction. This effecthas also been observed" in the lanthanide trihydroxideswhere the Ln-OH stretching and Ln-0-H defor-mation frequencies were found to increase monotonic-ally from La to Dy with the atomic number of the rareearth element. The trend is very marked in the P-0stretching vibrations, still present in the 0-P-0bending vibrations and slight in the lattice vibrations.The crystal radii of the lanthanide ions in most crystal-line compounds have been found to decrease17 withincreasing atomic mass of the ion. Crystal radii areplotted in Fig. 6. This lanthanide contraction results in acloser packing of the ion groups as one proceeds

In the region of the

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G. M. BEGUN, G. W. BEALL, L. A. BOATNER AND W. J. GREGOR

66566 065565 064 564 0635

-------

E 625Q 62 0

615585 5 8 0 175570

49485 o / 448047547046 5

l o , I

57 59 61 63 65 67 69 71Atomic number

Figure 8. Bending modes ofearth orthophosphates. v. atomic number for rare

f rom L aP 04 to L uP 0 4 . The compression of theion then shortens the P-0 distance yieldinghigher stretching and bending frequen cies for the higher

1953o 0 O o o o o o 1- 185 E4 180-

A +A A A AA AA30 2 2 A 2 281 2 5 t I I I I ,57 59 61 63 65 67 69 71

Atomic n u m b e rFigure 9. Lattice modes of ( P o p v. atomic number for rareearth orthophosphates.

atomic number lanthanide phosphates. The data showno evidence for a Ln-0 bond in the crystals.In summary, Ram an spectra have been obtained forall of the anhydrous lanthanide orthophosphates exceptPm P 04 and for YP 04 . These spect ra have been cor-related with the crystal radii and lanthanide atomicnumbe rs a nd systematic variations were established.AcknowledgementsResearch sponsored by the Division of Material Sciences, Office ofBasic Energy Sciences, US Department of Energy, under contractW-7405-eng-26 for Union Carbide Corporation. W.J.G. is a graduatestudent at the University of Tennessee, Department of Chemistry,Knoxville, TN 37916, supported by the Division of Chemical Sciences,US Department of Energy, under contract DE-AS05-76ER04447with the University of Tennessee (Knoxville).

REFERENCES1. L. A. Boatner, G. W. Beall, M. M. Abraham, C. B. Finch, P. G.

Huray and M. Rappaz, in The Scientific Basis for NuclearWaste Management, Vol. 11, ed. by C. J. Northrup, p. 289.Plenum Press, N ew York (1980).2. L. A. Boatner, G. W. Beall, M. M. Abraham, C. B. Finch, R. J.Floran, P. G. Huray and M. Rappaz, in The Managementof Alpha -Contaminated Wastes, IAEA-SM 246173, lnter-nationa l Atomic Energy Agency, Vienna, Austria (in press).3. I.Richman, J. Opt. SOC. m . 56, 1589 (1966).

4. A. N. Lazarev, N. A. Mazhenov and A. P. Mirgorodskii, lzvest.Akad. NaukSSSR Neorgan. Mater. 14,2107 (1978).5. E. N. Yurchenko, E. B. Burgina, V. I.Bugakov, E. N. Muravev,V. P. Orlovsk ii and T. V. Belyaevskaya, Izvest. Akad. NaukSSSR Neorgan. Mater. 14,2038 (1978).6. R. J. Elliott, R. T. Hanley, W. Hayes and S. R . P. Smith, Proc. R.SOC. ond. Ser. A. 328,217 (1972).7. A. Hezel and S. D. Ross, Spectrochim. Acra 22.1949 (1961).8. T. F. Tenisheva, T.M. Pavlyukevich and A. N. Lazarev, Izvest.Akad. Nauk SSSR Seriya Khimich. No. 10, 1771 (Oct. 1965).9. A. Armbruster, J. Phys. Chem. Solids 37, 321 (1976).

10 . K. .Petrov, I. V. Tananaev, V. G. Pervykh and S. M. Petrush-11 . R . S. Feigelson, J. Am. Ceram. SOC. 7, 257 (1964).12 . R . W. G. Wyckoff, Crystal Structures, 2nd Ed., Vol. 111, pp. 1513 . I . Krstanovic, 2. ristalogr. 121, 315 (196 5).14 . P. Dawson, M.M. Hargreave and G. R . Wilkinson, J.Phys. C.15 . G. W. Beall, L.A. Boatner, D. F.Mullica and W. 0.Milligan, J.16 . F. Weigel, V. Scherer and H. Henchel. J.Am. Ceram. SOC. 8,17. R. D. Shannon and C. T. Prewitt, Acta Crysrallogr. Sect. 8 5,18 . B. I. Swanson, C. Machell, G. W. Beall and W. 0. Milligan, J.

kova, Zh. Neorg. Khim. 12, 2645 (1967).

and 33. Wiley-lnterscience, New York (1965).

4, 240 (1971).Inorg. Nucl. Chem .43, 101 (1981 ).383 (1965).925 (1969).Inorg. Nucl. Chem . 40,694 (1978).

Received 26 November 1980@ US Government, 1981

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