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1412 Notizen

The Crystal Structure of Cesium Fluorotrioxochromate(VI), CsCr03FPeter Rögner, Klaus-Jürgen Range*Institute of Inorganic Chemistry,University of Regensburg, Universitätsstr. 31, D-93053 Regensburg, Germany

Z. Naturforsch. 50b, 1412-1414 (1995); received April 24, 1995

Cesium Fluorotrioxochromate(VI),Crystal Structure

The crystal structure of C sC r03F was deter­mined from single-crystal X-ray data as tetrago­nal, space group FVamd, with a = 5.7429(3), c = 14.505(1) A and Z = 4. The structure was refined to R - 0.028, R w = 0.022 for 307 unique re­flections.

In contrast to the earlier literature C sC r03F is not isomorphous with scheelite but with a- C sR e0 4. The structure contains isolated C r03F tetrahedra which are linked together by twelve­fold coordinated Cs= ions. The average C r -0 ,F distance is 1.633(4) A.

Among the fluorotrioxochromates MIC r0 3F the salt with M 1 = K crystallizes with the tetragonal scheelite-type structure (space group I4j/a), those with M 1 = Rb,NH4 are isotypic with B aS 04 [1]. For the cesium compound, C sC r03F, Ketelaar and Wegerif [2] found a tetragonal unit cell with a = 5.715(5) and c = 14.5(1) A. They also postulated space group I4j/a and hence the scheelite structure for C sC r03F. Such a morphotropic relation schee­lite —* barite and back to scheelite again with increasing cationic radius of the M 1 cations would be quite unique and difficult to understand.

During our investigations on perrhenates [3] we recently refined the crystal structure of a-C sR e04, the high-temperature modification of cesium per- rhenate [4]. The tetragonal lattice constants are a = 5.9607, c = 14.446 A and c/a = 2.423. In con­trast to the earlier literature [5], in which space group \Axla (and therefore a scheelite type struc­ture) was assumed for a-C sR e04, we could con­firm the space group I4j/amd to be the right one.

Another compound which crystallizes in the a- C sR e04 structure type is cesium perbromate, C sB r04 [6] with unit cell parameters a - 5.75, c =

* Reprint requests to Prof. K.-J. Range.

14.82 A and c/a = 2.58. Though the crystal struc­ture was originally described in space group I4x/a, we found that space group Mj/amd is appropriate for C sB r04 [7].

The same is true for cesium fluorotrioxosulfa- te(VI), C sS03F {a = 5.6317, c = 14.1991 Ä, c/a = 2.521) which crystallizes with the a -C sR e0 4 struc­ture (space group I4j/amd, [8]) and not with the scheelite structure (space group R ^ a ) as pre­viously postulated [9].

Looking at the axial ratios c/a, there is a definite difference between K C r0 3F (c/a = 2.371 [1] and C sC r03F (c/a = 2.539 [2]). Regarding the similari­ties between a -C sR e0 4, C sB r04, C sS03F and C sC r03F with respect to the unit cell dimensions a scheelite type structure of C sC r0 3F became questionable. We therefore decided to reinvesti­gate the crystal structure of C sC r03F with modern single-crystal methods.

Cesium fluorotrioxochromate was prepared by dissolution of cesium dichromate (prepared from C r0 3 (puriss., Merck) and Cs2C 0 3 (p.a., Fluka)) in hot hydrofluoric acid (38-40% , puriss., Merck) and subsequent slow cooling of the clear, orange- coloured solution to room temperature. A tetrago- nal-bipyramidal crystal of C sC r03F (crystallo­graphic forms {112}, {001} and {101} ) with approxi-

Table I. Crystal data, data collection and refinement parameters for CsCrC^F1.

Formula CsCrO^FMolecular weight 251.90 g.m ol- 1Space group 14,/amd (No. 141)Cell dimensions a = 5.7429(3) A

Z

c = 14.505(1) Ä d a = 2.5257(2) V = 478.39(4)4

F(000) 448Temperature 296(1) KRadiation M oKa (X = 0.71073 A)Scan mode co-2 <9Scan width (0.7 + 0.35 tan <9)°2 0 range 2° < 2 0 < 70°/i Ac /-limits - 9 < / z < 9 ; 0 < / t < 9 ; 0 < / < 2 3(sin 6 >/A)max 0.81 A " 1

Recorded reflections 1290Unique reflections, /?jnt 307, 0.022Reflections used in least 307

squares refinement Parameters refined 13(-̂ /oOmax 0 . 0 0 1

Extinction coefficient g 4.8(1).10' 7

Final R, R tv 0.028,0.022( l̂(?)max. min +0.5, -0 .6 e .Ä ”3

a Here, as in the following tables, the standard devia­tions are given in parentheses.

0932 - 0776/95/0900-1412 $06.00 © 1995 Verlag der Zeitschrift für Naturforschung. All rights reserved. D

Notizen 1413

mate dimensions 0.11 x 0.10 x 0.08 mm3 was used for the structure determination.

Precession photographs exhibited the Laue class 4/mmm and the reflection conditions (h k l): h + k + I = 2 n, ( h k 0): h, k = I n and ( h k l ) : 2/z + I = An which uniquely indicated the space group Rj/am d. A scheelite type structure could therefore be ex­cluded with certainty.

Data collection was performed on an Enraf- Nonius CAD-4 diffractometer using M oKa radia­tion (graphite monochromator in incident beam). The unit cell parameters were obtained by a least- squares refinement based on 23 carefully centered reflections in the range 9.8 < 0 < 18.6°. Three standard reflections were measured every 100 min, indicating only random fluctuations in intensity. After reduction of the 1290 recorded data a set of 307 independent reflections with I > 0a(I) re­mained (Rint - 0 .022), of which all were used in the subsequent calculations. Crystallographic and experimental data are summarized in Table I.

All calculations were carried out using the pro­grams SHELX-76 [10] and SHELXS-86 [11]. Atomic scattering factors and corrections for anomalous dispersion were taken from the In­ternational Tables for X-ray Crystallography [12],

The structure was solved by Patterson methods, followed by successive difference Fourier synthe­ses. The final full-matrix least-squares refinement (including anisotropic displacement factors and an extinction correction of the form Fcorr = Fc(l - gFc2/sin 0 ) converged at R = 0.028 and R w - 0.022. Atomic positions and displacement factors for C sC r03F are given in Table II, derived atomic dis­tances and angles in Table III*.

The results of the structure refinement show that C sC r03F and a -C sR e0 4 are indeed isostruc- tural. Therefore, the oxygen and fluorine atoms are statistically distributed on the 16 h site.

* Further details of the crystal structure determination can be obtained from the Fachinformationszentrum Karlsruhe GmbFl, D-76344 Eggenstein-Leopoldsha- fen, by quoting the depository number CSD 59025.

Table II. Atomic positional parameters and displace­ment factors [A 2] for C sC r03F.

Fractional atomic coordinates and equivalent isotropic displacement parameters a.Origin at centre (2/m)

Atom Site x/a y/b z/c u eq

Cs 4a 0 3/4 Cr 4b 0 1/4 O.F 16h 0 0.5139(6)

1/83/80.6877(3)

0.0298(1)0.0309(3)0.072(2)

Anisotropic displacement factors

Atom U n U 22 u 33 U 23

Cs 0.0320(1) U n Cr 0.0326(3) U „O.F 0.066(2) 0.076(2)U 12 = u 13 = 0

0.0254(2)0.0274(5)0.074(2)

000.044(2)

a U eq is defined as one third of the trace of the ortho- gonalized Ujj tensor.

Table III. Selected interatomic distances [A] and bond angles [°].

C r-O .F 1.633(4) (4x) O .F -C r-O .F 112.3(2) (2 x)O .F-O .F 2.643(5) (4x) O .F -C r-O .F 108.1(1) (4x)O .F-O .F 2.712(5) (2 x)

C s-O .F 3.111(4) (4x) C s-C r 4.0608(2)C s-O .F 3.372(3) (8 x) C s-C s, C r-C r 4.6254(3)

The structure of C sC r03F consists of isolated C r0 3F tetrahedra which are linked together by ce­sium ions. Cesium is twelvefold coordinated by ox­ygen and fluorine with an average C s -0 ,F dis­tance of 3.285(3) A. The C r -0 ,F distance of1.633(4) A is in agreement with the mean values of 1.648(5) A in K C r03F and 1.633(7) A in R b C r0 3F [1].

AcknowledgementsThe generous support given by the D eut­

schen Forschungsgemeinschaft (Graduiertenkol­leg “Complexity in Solids - Phonons, Electrons and Structures”) and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank Dr. U. Klement for the collection of diffracto­meter data.

1414 Notizen

[1] W. Granier, S. Vilminot, J. D. Vidal, L. Cot, J. Fluo­rine Chem. 19, 123 (1991).

[2] J. A. A. Ketelaar, E. Wegerif, Rec. Trav. Chim. Pays- Bas 58, 948 (1939).

[3] P Rögner, Dissertation, University of Regensburg (1993).

[4] K.-J. Range, P Rögner, A. M. Heyns, L. C. Prinsloo, Z. Naturforsch. 47 b, 1513 (1992).

[5] H. Beyer. A. Müller, B. Krebs, Z. Phys. Chem. 234. 423 (1967).

[6 ] E. Gebert, S. W. Peterson, A. H. Reis, E. H. A ppel­mann. J. Inorg. Nucl. Chem. 43. 3085 (1981).

[7] P. Rögner, U. Schießl. K.-J. Range, Z. Naturforsch. 48 b, 235 (1993).

[8] P. Rögner, K.-J. Range, Z. Naturforsch. 48 b, 688 (1993).

[9] H. Seifert, Z. Kristallogr. 104, 385 (1942).[10] G. M. Sheldrick, SHELX-76. A program for the so­

lution of crystal structures. Univ. of Cambridge, England (1976).

[11] G. M. Sheldrick, SHELXS-86 . A program for crystal structure determination, Universität Göttingen(1986).

[12] International Tables for X-ray Crystallography, Vol. IV. The Kynoch Press, Birmingham (1974).