An 121Sb Mössbauer study of metal antimonates

I. more. m;tl. ( h e m .

1974. Vo[ 36, pp. 2177 2183. Pergamon Press. Printed in Great Britain

AN 1218b MOSSBAUER STUDY OF METAL ANTIMONATES* J. B. WOOTEN,[ G. G. LONG + and I. H. BOWEN Department of Chemistry, North Carolina State University, Raleigh. N.C. 27607

(First received 3 August 1973: in revised form 10 December 19731

Abstract--A series of metal antimonates crystallizing in the rutile (M(III)SbO4), trirutile or lead antimonate (M(lI)SbzO 0, and pyrochlore or weberite (M2llllSb2OTt structures have been studied by ~ZISb M6ssbauel spectroscopy. All of these structures contain SblV)O 6 units which differ slightly but are essentially octahedral. The observed line widths are correlated with either distortion of the octahedra or the presence of multiple sites for antimony. The isomer shifts with respect to lnSb were found to fall in the [ollowing ranges: pyrochlores, 9.14~9-36 mm/sec: trirutites 8.65-8-99 mm/sec: rutiles 8.35 9.02 mm/sec: weberites, 8-74 8-82 mm/sec: lead antimonates, 8.33 8-75 mm/sec. The most striking effect is the remarkable variation of over 1 mm/sec in isomer shift in compounds in which the local environment is basically the same. This variation is related not only to structure type but also to the nature of the positive ion,

INTRODUCTION Tin: STFe isomer shifts (I.S.) from M6ssbauer spectra of inorganic compounds in which the iron has the same oxidation number, the same number and kind of nearest neighbors, and nearly the same geometric arrangement of these nearest neighbors are essentially identical[l]. The same may be said with regard to ~19Sn[2~. The interpretation of M6ssbauer spectra of a number of series of such compounds, e.g, Fe(ll) fluorides with the futile structure[la], Fe(llI) oxides having the pyrochlore structure[lb~, and rare earth stannates[21, has depended on parameters other than the essentially identical isomer shifts, i.e. width. intensity and quadrupole splitting. Indeed, the similarity of the isomer shift to those of other octahedral compounds has been used to establish the identity of the FeCI36- ion[3a], although a small difference (0.007 mm/sec) was noted between crystalline modifications of Cs3FeCl6[3b]. Since 121Sb isomer shifts are spread over a relatively large range, Sb compounds seemed to us to be likely ones to exhibit differences in isomer shifts corresponding to small differences in environment of the M6ssbauer nucleus. The isomer shift variation for ~21Sb is about 5-4 times greater than that for ~19Sn in electronically similar series~4], whereas

* Presented in part at the Southeastern Regional American Chemical Society Meeting, Charleston, S.C., Nov. 7-9, 1973 t Present address: Department of Chemistry, Clemson University, Clemson, S.C. :~To whom correspondence should be addressed.

the natural line width is only 3-3 times greater. Therefore, the isomer shift is, in principle, a more sensitive parameter for antimony than for tin compounds, A comparison with STFe is not so readily made, since the electronic structures are so different for the two elements. However, the natural line width is about 11 times greater for 121Sb, so a 0-1 mm/sec I.S. difference for SVFe would be about the same fraction of the line width as a 1 mm/sec difference for 121Sb. In any event, Veits et a/.{5] recently reported that the alkali metal cation affects the magnitude of the 121Sb isomer shift in trithioantimonateilll) and triselenoantimonate{lll) compounds, which was ascribed to an increase in ionic character of the Sb-S {or Se) bond with increasing cation radius. For the above reasons we have examined and report herein the 121Sb M6ssbauer spectra of a number of antimony{V) compounds of known structures in which the immediate neighborhoods of the Sb(VI sites are very similar. Although the compounds have three different stoichiometries M(III)SbO4, M(ll)Sb20~ and M(II)Sb20 7, and five different lattice types, the SbIV), in each case is octahedrally surrounded by six oxygens. In the lead antimonate (C312) lattice there is a single antimony site at the center of an almost regular oxygen octahedron (Sb-Ot distance is 2.00 A in PbSb206[6] although the site symmetry is less than cubic when further neighbors are considered. In the trirutile structures there is only a single antimony site, but the octahedron of oxygens is distorted (Sb-O distances of 2.00 and 198A. were observed in ZnSb20~[7]). The octahedra should be similarly distorted in those compounds having the futile structure, but since the SbIV)

2177

Cream Beige White White

6 9 8,9 9

L P W W

PbSb206 Cd2Sb207 Ca2Sb20 ~ Sr2Sb207

A A A A

B~ A A A A B B A A A A A PbO + Sb203 2CdO + Sb20 3 2CACO3 + Sb203 2SRCO3 + Sb2Os

AIC13 . 6H20 + KSb(OH)6 Cr203 + Sb203 Fe203 + Sb2Oa MgO + Sb203 ZnO + Sb20 3 CoC12 . 6H20 + KSb(OH)6 NiC12.6HzO + KSb(OH)6 CuO + Sb203 CaCO3 + Sb2Oa SrCO 3 + Sb20 3 BaO + Sb203 CdO + Sb203

Reagents

24/950, 24/1150 24/750, 55/1150 24/1300 24/750, 30/1000 4/600, 10/900, 24/950 24/950, 24/1300 24/950, 24/1300 24/750, 48/1000 10/900, 24/950, 24/1000 20/975, 50/1000 20/975, 41/1000 20/650, 124/750, 48/850 72/900, 27/10001l 20/450, 165/750 48/750, 20/900, 72/1000 28/1200 28/1200

Firing schedule? (hr/temp.)

1 1 1 2

1 0 3 1 0 0 0 0 2 1 1 1

No. of HCI extractions

* R = rutile, T = trirutile, L = lead antimonate (C312), P = pyrochlore and W = weberite. t The times listed are the actual times the samples were in the furnace. These frequently were just a matter of convenience and shorter times might well be adequate. :~ Several attempts to prepare A1SbO4 by method A failed. § This actually is a deformed trirutile structure. See Ref. [7]. HThe complicated heating schedule seems to be necessary to avoid excessive volatilization of Sb20 3.

White Tan Red-brown Cream Gray Orange Pale green Green-yellow Cream White White Pale green

10 10 10 7 7 7 7 7 6 6 6 6

R R R T T T T T§ L L L L

A1SbO4 CrSbO4 FeSbO4 MgSb206 ZnSb206 CoSb206 NiSb206 CuSb206 CaSb206 SrSb206 BaSb206 CdSb206

Color

Ref.

Structure*

Formula

Method of prep.

Table 1. List of the antimonates prepared along with some properties of the compounds and reaction conditions

©

e~

o

z

O ©

2179

~2~Sb M6ssbauer spectra of antimonates a n d M(III) are r a n d o m l y distributed[10], there are a n u m b e r of very nearly e q u i v a l e n t Sb sites. In p y r o c h l o r e s there is a single a n t i m o n y site, but evidence as to w h e t h e r or not the o c t a h e d r o n is d i s t o r t e d is inconclusive[8]. In rare earth p y r o c h l o r e s of f o r m u l a M 2 S n z O ~ , the 119Sn M 6 s s b a u e r spectra s h o w q u a d r u p o l e splitting, indicative of n o n - c u b i c s y m m e t r y at the tin site[3a]. T h e weberites have equal n u m b e r s o f two s o m e w h a t different o c t a h e d r a l a n t i m o n y sites (in C a 2 S b 2 0 7 one site has six oxygens at 1.95-1.96A. from the Sb and the other site has SI:~O distances of 1.96 a n d 2.00AI9]).

EXPERIMENTAL

Preparation and characterization o[ lhe antimonales All of the antimonates studied have been previously reportedI6 10], but details of preparation are frequently meager in the literature. In this study the compounds were prepared by one of the following two methods and are listed along with some of their properties and details of preparation in Table 1. Method A, similar to the procedure described by Butler et al.[111, consisted of firing in an electric furnace an intimate, finely ground mixture of the theoretical amount of reagent grade metal oxide or carbonate with Sb20~ in an open, fused alumina crucible. The firing schedules are listed in Fable 1 ; the final firing temperature (highest) was normally determined on the basis of reports in the earlier literature [16-10]. Afler each intermediate temperature firing, the sample was allowed to cool and reground before firing at the next high temperature. After firing the sample at the highest temperature indicated in the firing schedule, the X-ray powder pattern was determined and compared either ~ith reported d-spacings (or sin20 values) or compared x~ith d-spacings calculated from reported unit cell dimensions. The product was not considered to be satisfactory for further study unless the X-ray powder diffraction lines agreed in positions and intensities with those expected (or reported) for the sample, and extraneous lines (if any) had very low intensities. Common impurities were frequently identified as the metal oxide or Sb20¢. These impurities could frequently be removed by finely grinding the product, stirring it in a beaker with hot conc. HCI for 1 2 hr, filtering off the solid and washing it well with water, drying, and firing it again in the furnace at the highest temperature listed in the firing schedule. The X-ray diffraction pattern was then redetermined. In some cases the extraction procedure had to be repeated two or three times to remove oxide impurities. Method B consisted of precipitation of a hydrated metal antimonate by mixing a solution of the metal chloride with a solution of potassium antimonate. The finely divided precipitate was separated by filtration, washed repeatedly with water, dried at I10°C, and fired according to the schedule given in Table 1. The X-ray powder patterns were checked as indicated under Method A, and in the case of AISbO 4 it was necessary to remove impurity lines by treatment with hot hydrochloric acid, vide supra. The compounds CoSb206 and NiSb206 when prepared at 950°C are brownish colored powders and have the trifutile structure. When heated to higher temperatures, e.g. 1300°C, some antimony oxide apparently is volatilized, a crust of cobalt or nickel oxide forms, and in the case of nickel a pale green solid (in the case of cobalt, an orange

solid) may be separated. The powder patterns for these solids are essentially identical to the respective brownish compounds except that the lines are sharper and the signalto-noise ratio is improved. In the case of nickel, a powder pattern has been reported for a trirutile that is said to have a non-stoichiometric composition [12]. ~l&~sbauer ~pectra Spectra were determined at liquid nitrogen temperature using equipment and procedures essentiall~ as described previouslyI13, 161. Absorbers were prepared by mixing the powdered compound with powdered polyethylcnc in a flat polyethylene holder. Spectra were taken for at least 30 hr using Ni21Sn2B6(12tSb) as the source. Using both the escape and photo peaks from a xenon methane proportional counter, 60 250 thousand counts per folded velocity point were accumulated. Duplicate determinations were run l~,~r most compounds. Isomer shifts have been converted to lnSb reference by adding 1.68mmsec to the measured valuesI131. The spectra were fitted by (a) a single Lorentzian peak, (b) eight-line quadrupole splitting both with positive and negative eeqQ as input and tc) lor the weberites a two peak Lorentzian fit was attempted due to the known presence of two equally populated antimony sites. A quadrupole lit to the spectral data for Ca2Sb20, is shown in Fig. 1 It is interesting to note that the quadrupole splitting is not particularly evident to the eye even though there is a marked improvement in Z2 for the quadrupole fit with respect to the fit to a single Lorentzian. The spectra of all of the compounds are very similar in appearance to this one, althongh quadrupole splitting did not always improve the fit.

RESULTS AND DISCUSSION M 6 s s b a u e r data for the a n t i m o n a t e s are reported in Table 2. Intensity, w i d t h at half m a x i m u m a n d isomer shift are given for a single L o r e n t z i a n fit to the spectral d a t a for each c o m p o u n d . Large widths ( > 3-4 ram/see), observed in a n u m b e r of the spectra, may be caused by _

~,..=~

2z~_~,

+

+~

.~

:,=

o

Velocity,

mm/sec

Fig. 1. M6ssbauer spectrum of Ca2Sb20~. This particular fit has an isomer shift of 6-97 mm/sec with respect to the source, a width of 2.9 mm/sec and e2qQ of 5.7 mrn/sec. To convert the isomer shift to the basis of lnSb it is necessary to add 1-68 mm/sec, thereby giving 8.65 mm sec.

J.B. WOOTEN,G. G. LONG and L. H. BOWEN

2180

Table 2. M6ssbauer parameters* Sample Intensity thickness (~) Compound (mgSb/cm 2) (_+3)

Width at half maximumt (mm/sec) (_+0.1)

I.S. (isomer shift) (mm/sec)(_+O.1)

Rutiles AISbO4 FeSbO4§ CrSbO4

10 10 10

14 17 12

3.8 3.6, 3.0 3.8

9.02 8.43 8.35

Trirutiles MgSb206 NiSb206l] CoSb206

10 10 10

12 22 14

CuSb206

15

15

ZnSb206

15

17

3.6 4.1, 3.4 3.4 3-7 3.8

8.99 8.95 8.65 8.70 8.82

16 14 14 14 12

3-3 3-4 3.3 3.3 3.3

8.60 8.75 8.48 8.52 8.33

Lead antimonate (C312) CaSb206 10 SrSb206 10 CdSb206 10 BaSb206 10 PbSb206 10

e2qQ (quadrupole splitting) mm/sec(_+0-5)

Improvement in fit (~o)

:~ 6.1 :~

17

-5.5

20

Pyrochlore

Cd2Sb207

10

10

3-6

9.36

:~

Weberites SrzSb207 Ca2Sb207

10 10

21 19

4.2, 3.2 3-7, 3.0

8.82 8.74

7.4 5.9

27 65

* Isomer shifts are reported relative to lnSb. The numbers under the headings in parentheses are errors which have been estimated on basis of duplicate determinations of each spectrum. Thus these numbers are somewhat higher than those frequently reported based on the computer fit to a single spectrum. t The fit with quadrupole splitting is reported (in italics) in those cases where a significant improvement in Z2 is attained over fitting the data to a single Lorentzian. Percent improvement in fit (ZL2...... -- Z ~ d ) × 100

.~2orentz When X2 is markedly decreased by permitting quadrupole ~plitting in the fitting procedure, width at half maximum also is decreased and is more nearly the theoretical width. :~ In these cases there was a narrowing of widths and a small improvement in X2 (less than 20 per cent) when the data were fit allowing quadrupole splitting. This amounted to 4 or 5 mm/sec, but equally good fits were obtained with the sign of e2qQ positive or negative. The data reported, therefore, are for fitting to a single Lorentzian. The isomer shifts for both fitting procedures is within the experimental error for the values listed in this table. § 57Fe M6ssbauer parameters were determined at room temperature for this compound in a sample containing ~ 8 mgFe/cm 2 and found to be I.S. = 0.40 + 0.01 mm/sec (relative to metallic iron), Q.S. = 0.76 +_ 0.02 mm/sec and a width at half maximum of 0-38 + 0-2 mm/sec for each peak of the doublet. This compound has been reported in the literature [14] as having I.S. = 0.41 + 0.02 and Q.S. = 0-78 + 0.06 mm/sec for S7Fe and I.S. = 0.0 + 0.2mm/sec for 121Sb relative to SnO2(121Sb) source. Relative to InSb [15] the latter value corresponds to 8.5 mm/sec. Thus, both iron and antimony data are in excellent agreement. II Probably non-stoichiometric. See ref. [12]. a multiplicity of similar sites a n d / o r quadrupole splitting. Since the a n t i m o n y sites in all of these compounds have less than cubic symmetry, a quadrupole fit was tried for each set of spectral data, but is reported (Table 2, in italics) only for those c o m p o u n d s where Z2 values and other M 6 s s b a u e r parameters were significantly improved over the single Lorentzian fits. Indeed, those spectra which definitely show quadrupole splitting belong to c o m p o u n d s which have crystal structures in which the a n t i m o n y nearest neighbors are at different distances. Q u a d r u p o l e splitting, however, was not observed in all of the c o m p o u n d s studied which have rutile, trirutile or weberite structures. This could

be due to the lack of our ability to resolve small e2qQ in spectra of lower intensity (i.e. AlSbO4 and CrSbO4), or in the case of the rutiles, due to the fact that the spectral peak is made up of contributions from many, slightly different, a n t i m o n y sites. In any event, quadrupole splitting undoubtedly contributes to broadening of the 1215b M 6 s s b a u e r peaks in the rutiles, trirutiles and weberites since the observed peaks are too wide and the oxygen octahedra are k n o w n to be distorted [7, 9, 10]. All of the metal atoms in trirutiles and in rutiles are located in equivalent sites; in the trirutiles there is an ordered placement of the two metals with respect to

121Sb M6ssbauer spectra of antimonates one another, in the rutiles the placement is random. Thus, the local symmetry of the antimony site should be the same in both lattice types. In the 57Fe M6ssbauer spectrum of the rutile, FeSbO4, narrow peak components with definitive quadrupole splitting are observed (Table 2, Note §). This means that although there is random occupation of metal sites by iron, the iron nucleus experiences only one type of environment. The same should be true for antimony which randomly occupies the same type of site, and indeed, quadrupole splitting is observed. The a21Sb quadrupole splitting determined for FeSbO4 has a positive sign, that for NiSb20 6 a negative sign. The latter, however, is probably not characteristic of the trirutiles since the sample showing the negative quadrupole splitting was heated at 1300°C, and very likely is non-stoichiometric [121 which could markedly affect the shape of the M6ssbauer peak. A preparation fired at 950°C gave less well-defined X-ray diffraction peaks and a a2aSb M6ssbauer spectrum with essentially the same isomer shift, but of lower intensity and no resolvable quadrupole splitting. The two weberites gave peaks of relatively high intensity. An attempt was made to fit the data for each of the compounds to two Lorentzian peaks, since the weberites are known to have two equally populated antimony sites. A good fit could be obtained only by having intensities of the two peaks differ by a factor of three and with isomer shifts that differed by 1.7 mm/sec. Since these are not tenable parameters, the two weberite sites must have very similar isomer shifts, with the broadening due to quadrupole splitting. Indeed, both compounds gave resolvable quadrupole splitting and in neither case was there an indication from the spectra of a difference in e2qQ between the sites. The single pyrochlore reported, Cd2Sb2Ov, has a line width of 3.6 mm/sec which indicates that quadrupole splitting is likely. Previously we have reported negative quadrupole splitting for Group I pyrochlores [16]. A negative e2qQ for Cd2Sb2Ov of - 5 mm/sec does fit the spectrum, but shows only marginal improvement over the single Lorentzian peak fit. Of the compounds studied those with the lead antimonate structure have the narrowest M6ssbauer peaks consistent with the reported symmetry[6] of the oxygen octahedra. The peaks are broader however, than for InSb, for example, which has cubic site symmetry. A given increase in isomer shift is consistent with a decrease in s-electron density or to a more marked increase in p-electron density at the antimony nucleus [4]. If the antimony atoms were truly octahedrally surrounded by oxygens in these compounds, the observed differences in isomer shifts would have to be related either to changes in s-electron density effected by different metal ions or by differences in S b - O distances demanded by the particular structures. Since there are significant distortions in the oxygen octahedra around antimony in some of the compounds, p-electron density cannot be considered overall as entirely constant: it

2181

9.4

9.2

t [] •,,

[]

9.c

L~

i

C312 v Rutile o Trirutile Pyrochlores Weberite

~Transition ! metals

.%%... .%

8.E E 8.E

"", ,

/ Representative l"-..~lement s

8.4-

8.2

L

I

I

50 I00 Atomic weight

200

Fig. 2. Plot of ~21Sb isomer shift vs the atomic weight of the metal for a number of metallic antimonates.

would be most nearly so for compounds having common structure types. Isomer shifts for the antimonates reported in this paper as well as those for the pyrochlores, sodium, potassium and silver antimonate[16]* and Sb60~ 3117] previously reported, fall within a 1 mm/sec range (Table 2).t It is evident that the pyrochlores O.149.36mm/sec) are at the high end of the range, the trirutiles (8.65-8.99 mm/sec) fall more in the middle of the range and that compounds with the lead antimonate structure (8.33-8.75 mm/sec) occur at the lower end of the range. The above isomer shift limits are not exclusive for a particular crystal type, as the trirutile isomer shifts slightly overlap those for the lead antimonate structures, the two weberites are essentially identical and occur with the trirutiles, and the rutiles seem to fall throughout the 1 mm/sec range. In two of the instances where there are two compounds made up of the same elements, but of different formulas and structure types (CaSb206(trirut) and CazSbzOT~wob~: SrSb2O6(trirut ) and Sr/Sb2Ov(web)), the respective isomer shifts are the same within experimental error. In the third instance (CdZb206(c31z i and Cd2Sb zOT(py,o)) the isomer shifts are markedly different. Thus, it would seem that the Sb (specifically, its s-electron density) in the corresponding trirutiles and weberites

* These pyrochlores are variable in composition and may contain both Sb(III) and Sb(V). The following data were reported: sodium antimonate--average I.S. = 9.27 mm/sec, average e2qQ= - 4-7 mm/sec; silver antimonate average l.S. = 9.17 mm/sec, average eZqQ = - 5.6 mm/sec: potassium antimonate--l.S. = 9.22 +_ 0-05 mm/sec e2qQ = -5-3 _+ 1 mm/sec. t The I.S. values listed in Table 2 are for a single Lorentzian fit to the data. In no case does fitting of data to include quadrupole splitting cause the IS. value to be outside the errors indicated in the Table.

2182

J. B. WOOTEN,G. G. LONGand L. H. BOWEN

+

.o,~.-°°°°°

921j A0

l+3Iom Mg~/

m 9"0

~AL

E E

~

Zn

Sr

8"8 .¢ J:: ~n

Sr

Ca

Cu

8"6

8"4

8";

~'06 0"1

0"2

04 0"6 0"8 I'0

z/(-~,~ r3),

2~0

4"0

6"0

~-~

Fig. 3. Plot of 121Sb isomer shift vs charge density [Z/4~r 3 where z is the charge on the ion and r is the six-coordinate ionic radius as given by R. D. Shannon and C. T. Prewitt, Acta crystallogr. B26, 1076 (1970)] of the metal ion in a number of metal antimonates. The lines associated with the -- 1 and the + 2 cations have been rather arbitrarily drawn (see text). Crystal structure types are indicated as follows: pyrochlores, r-l; weberites, II; rutiles, ©; trirutiles, @: lead antimonates, A. is very similar, while Sb in lead antimonates is somewhat different from that in the pyrochlores (s-electron density at Sb is less in the pyrochlores than in the lead antimonates). Upon further examination of the isomer shift data, it was noted that the antimonates of the lighter elements (Na, A1, Mg) have the more positive isomer shifts (~9mm/sec), while some of the antimonates of the heavier elements have relatively low isomer shifts (e.g. PbSb206, I.S. = 8.33 mm/sec). A semi-logarithmic plot of 121Sb isomer shift vs atomic weight of the cation (on the log scale) was prepared (Fig. 2). It is evident from this plot that the isomer shifts of the rutiles, trirutiles, lead antimonates and weberites fall roughly along two lines: (a) representative elements show a decrease in isomer shift with increasing atomic weight, and (b) the first row transition elements show an increase in isomer shift with increasing atomic weight. However, within experimental error, the pyrochlore isomer shifts are independent of the atomic weight of the cation. It is certainly possible that cation atomic weight is not the best parameter to which isomer shifts of these antimonates should be related. The fact that the slope of the transition element line is opposite that of the representative element line suggested that a parameter involving radius or volume of the cation might give an interesting relationship. A number of such plots were prepared, but possibly the most interesting one was a plot of I.S. vs charge density on the cation, Fig. 3. With the exception of the cadmium pyrochlore the I.S. values are distributed along three different lines dependent

upon the charge on the cation. Particularly in the compounds with the di- and tri-positive cations, I.S. becomes more positive as the charge density on the cation increases. Although this is true also for the three compounds containing uni-positive cations, when one takes into consideration the errors involved in the isomer shift determinations, a horizontal line would be just as good as the one drawn. It is particularly interesting that all of the pyrochlores, including the one containing the + 2 ion, have isomer shifts that occur over a very narrow range. Other compounds of a common crystal type seem to fall in specific regions on the graph. Thus, the lead antimonates occur near the lower end, the weberites near the middle and the trirutiles near the upper end of the di-positive ion line and the rutiles occur along the tri-positive line. The lines probably are best construed as merely grouping charge types together. Indeed, the close relationship of this plot with respect to Fig. 2 is evident if the pyrochlores are neglected, and if instead of the di-positive and tripositive lines, transition metal and representative element lines are drawn. Certainly factors such as structure type and nature of the cation affect the scatter of the isomer shifts in these antimonates, but it is difficult to decide from the present data whether the isomer shifts are more sensitive to charge on the cation, mass of the cation, d-electron levels of the cation, or some other parameter or combination of parameters not yet considered. It is indeed evident that the isomer shift of antimony in these antimonates can be affected by the "relatively remote" cations. This probably is best interpreted in terms of an increase in charge density on the cation causing an increase in the ionic character of the Sb(V) due to loss of s-electron density to the octahedral array of nearest neighbor oxygens. In the relatively open framework of the pyrochlore, the cation has but little influence and hence the isomer shifts are essentially constant, characteristic of Sb(V) bound through six oxygens to six other antimonys (octahedra connected through points). In the lead antimonates the effect of the cations is not as pronounced as in rutiles and trirutiles because the metal ions are large with low charge densities and lie between layers of antimonate octahedra. The compounds which contain the very large cations and have the lead antimonate structure show isomer shifts which would essentially be characteristic of antimony bound through six oxygens to three other antimony atoms (octahedra bound through edges). Since these isomer shifts are more negative than those for compounds with octadedra connected through points (pyrochlores), the antimonate framework made of octahedra bound through edges is more covalent than the framework of octahedra connected through points.

Acknowledgements--This work was supported by the National Science Foundation Grant GP33516X. The authors also wish to thank Dr. H. H. Stadelmaier of the Engineering Research Department for making available the X-ray diffraction equipment.

2t Sb M6ssbauer spectra of antimonates REFERENCES

I. (a) O. Knop, F. Brisse, R. E. Meands and J. Bainbridge, Can. J. Chem. 46, 3829 (1968); (b) N. N. Greenwood, A. T. Howe and F. Menil, J. chem. Soc. (A), 2218, (1971). 2. Ca) H. M. Loebenstein, R. Silber and H. Zmora, Phys. Lett. 33A, 453 (1970); (b) L. M. Belyaev, I. S. Lyubutin, L. N. Dem'yanets, T. V. Dmitrieva and L. P. Mitina, Soviet Phys., Solid St. 11,424 (1969). 3. (a) G. M. Bancroft, A. G. Maddock, W. K. Ong and R. H. Prince, J. chem. Soc. (A), 723 (1966); (b) E. Frank and D. St. P. Bunbury, J. inorg, nucl. Chem. 34, 535 (1972). 4. S. L. Ruby, G. M. Kalvius, G. B. Beard and R. E. Snyder, Phys. Rev. 159 239 (1967). 5. B. N. Veits, S. 1. Berul', V. Ya. Grigalis and Yu. D. Lisin, lzv. Akad. Nauk SSR, Ser. Fiz. Tekhn. N. 1, 48 (1971). 6. A. Magn61i, Ark. Kemi Mineral. Geol. 15B, No. 3 (1941). 7. A. Bystr6m, B. H6k and B. Mason, Ark. Kemi Mineral. Geol. 15B, No. 4 (1941).

2183

8. F. Brisse, D. J. Stewart, V. Seidl and O. Knop. ('an. ,1 Chem. 50, 3648 (1972). 9. A. Bystr6m, Ark. Kemi Mineral. Geol. 18A, No. 2l (1944-5). 10. K. Brandt, Ark. Kemi Mineral. Geol. 17A, No. 15 (1943) 11. K. Butler, M. J. Bergin and V. M. B. Hannaford, ,/ electrochem. Soc. 97, 117 (1950). 12. A. C. Skapski, A.S.T.M. Powder Diffraction File, Card No. 17-619, American Society for Testing Materials, Philadelphia, Pa. 13. T. B. Brill, G. E. Parris, G. G. Long and L. H. Bowen. Inorg. Chem. 12, 1888 (1973). 14. F.-J. Ulrich, W. Meisel, J. Scheve and A. Y. Alexandrov, In Proc. Conj. M6ssbauer Spectroscopy. Dresden. 1971 (Edited by H. Schnorr and M. Kautz) p. 542 Deutsche Akademie der Wissenshaften zu Berlin, Zentralinstitut ftir physikalische Chemie, Berlin-Adlershof, D D R (1971). 15. G. G. Long, J. G. Stevens, R. J. Tallbane and [. H Bowen, J. Am. chem. Soc. 92, 4230 (1970). 16. L. H. Bowen, P. E. Garrou and G. G Long, .1. more nuel. Chem. 33, 953 (1971). 17. D. J. Stewart, O. Knop, C. Ayasse and F W D Woodhams, Can. J. Chem. 50. 690 (1972).