The crystal structure of Cs2W3010 (a = 16.103, c = 10.169 A, 14, Z = 120/11) was ... structure refinement [3], the composition of the latter compound is expressed ...
Journal of Structural Chemistry, Vol- 37, No. 4, 1996
CRYSTAL STRUCTURE OF THE STOICHIOMETRIC COMPOUND Cs2W3Ol0 WITH A FRACTIONAL NUMBER OF ATOMS IN THE UNIT CELL S. F. Solodovnikov, O. A. Man'kova, Z. A. Solodovnikova, N. V. Ivannikova, and V. I. Alekseev
UDC 548.736
The crystal structure of Cs2W3010 (a = 16.103, c = 10.169 A, 14, Z = 120/11) was determined from three-dimensional X-ray diffraction data (CAD-4 automatic diffractometer, 1612 reflections, R = 0.044). The structure has an A B 2 0 6 defective pyrochlore motif in which some of the groups consisting of four W06 octahedra have been subtracted from the tungsten-oxygen framework and substituted by Cs +. The crystal has no vacancies in the substitution site (as confirmed by the occupancy refinement), and the composition of the crystal is strictly stoichiometric with a fractional number of atoms in the unit cell. A similar composition is suggested for the isostructural rubidium compound, which was earlier considered nonstoichiometric.
In the course of investigations of the Cs2WO4-WO3 system [1], we confirmed formation of the Cs2W3010 compound, which has no pronounced homogeneity region according to XRPA data. Examination of these colorless isometric crystals of cesium polytungstate in a RKOP chamber (Mo radiation, Lane and oscillating crystal methods) showed that they correspond to the Cs22W320107 phase [2], whose structure is isotypical to the rubidium analog studied in [2, 3]. According to the structure refinement [3], the composition of the latter compound is expressed by the formula Rb2o+x(W406)l/3_x/12W3201os, suggesting the presence of a certain homogeneity region. To resolve this contradiction concerning the composition of cesium tritungstate we undertook an X-ray diffraction investigation of this compound. For structure determination we chose an isometric sample with an average size of about 0.2 ram. The parameters of the tetragonal unit cell (a = 16.103(2), c = 10.169(2) ~, V = 2536.9 ~3) were refined, and the intensities of 3277 reflections were measured ("Enraf-Nonius" CAD-4 automatic diffractometer, MoKa radiation, graphite monochromator, 09/20 scan mode, 20 < 60~ The intensities of the measured reflections were recalculated to structural amplitudes including absorption corrections estimated from transmission curves (u(MoKa) = 441.7 era-l); the calculations were performed on a PDP-11 computer with the SDP program package. Other structure calculations were accomplished with SHELXL-93 programs [4] using 1612 unique reflections with I > 2xr(/). Systematic extinctions (hkl : h + k + l = 2n) in the array of diffraction data satisfied three space groups:/4, 14, and 14/m. All further calculations were performed in space group/4, which was determined for the isotypical rubidium analog [2, 3], using the set of coordinates of heavy atoms from [2] as an initial approximation. The isotropic refinement of the set led to R = 0.120; the difference Fourier synthesis gave all 14 independent oxygen atoms and the W(5) defective site as shown by additional maxima in the vicinity of Cs(5) (the site was found earlier in [2]). The isotropic refinement of all atomic coordinates using the experimental weighting scheme yielded R = 0.082; anisotropic refinement including secondary extinction gave R = 0.044. In the final cycle of refinement, two partly defective oxygen positions, found from the Ap(xyz) synthesis, were added to the model of the structure; the sites were ascribed the same occupancy as for the coordinating W(5) atom. This minor addition did not alter the residual. The f'mal positional and Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhumal Struktumoi Khimii, Vol. 37, No. 4, pp. 750-755, July-August, 1996. Original article submitted September 27, 1995. 0022-4766/96/3704-0645515.00 9
Plenum Publishing Corporation
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TABLE 1. Positional and Thermal Parameters of Basic Atoms in the Crystal Structure of Cs2W3010 (standard deviations are given in parentheses) Atom W(1) W(2) W(3) W(4)
x
y
0.13601(7) 0.28315(6)
0.18627(6) 0.14757(6) 0.39321(6) 0.24046(7) 0.0494(7) 0 0
0.04092(6)
0(6) 0(7) O(8) 0(9)
0.01177(6) 0.0973(6) 0 0.5 0.4052(1) 0.2860(1) 0 0.1911(11) 0.3027(9) 0.1353(9) 0.1965(10) 0.0696(12) 0.0633(10) 0.2252(10) 0.3935(10) 0.2125(9)
0(10)
o.o97o(i )
0(11) 0(12) 0(13) 0(14)
0.4717(11) 0 0.3354(10) 0.1090(9) 0 0.047(9)
W(5)** Cs(1)
Cs(2) Cs(3)
cs(4) cs(5)*** 0(1) 0(2) 0(3) 0(4)
0(5)
O(15)** 0(16)**
0.2040(1) o.3883(1) o 0.0947(9) 0.0575(10) 0.4106(9) 0.1002(10) 0.1401(11) 0.1879(10) 0.2051(10) 0.2002(10)
o. 6(lO) 0.2999(11) 0.3411(12) 0.5
o.o144(lO) 0.4620(11) 0 0.119(9)
z 0.35745(13) 0.09620(12) 0.38162(13) 0.65726(12) 0.1220(14) 0.5 0.25 0.5016(2) 0.2251(2) 0 0.4209(21) 0.2368(19) 0.4943(21) 0.0309(24) 0.2434(23) 0.5059(25) 0.2359(22) 0.1869(21) 0.4885(21) 0.3154(21) 0.2603(24) 0.4395(27) 0.4943(21) 0.2436(20) 0.127(27) 0.019(21)
Ueq*
0.0104(4) 0.0105(3)
0.0067(2) 0.0114(3)
0.0046(29) 0.0381(16) 0.0318(11) 0.0249(6) 0.0194(4)
o.o6o6(lO) o.o11(4) 0.007(4) 0.005(4) 0.017(5) 0.019(5)
0.012(4) 0.011(4) 0.010(4) 0.008(4)
o.015(5) o.o18(5) o.o12(6) 0.007(4) 0.008(4) 0.000(42) 0.000(34)
*Uen = (Ull + 0"22+ U33)/3. **O'~ccupancyis 0.092(4). ***Occupancy is 0.908(4).
equivalent thermal parameters of basic atoms (anisotropic thermal parameters are available from the authors) are listed in Table 1, and selected interatomic distances are given in Table 2. The five independent tungsten atoms are characterized by typical octahedral coordination with average W-O distances of 1.91-1.94 .~. Each of the Cs(1)-Cs(4) atoms has 18 oxygen atoms, including "statistical" O(15) and 0(16), in the environment; for Cs(5), the coordination number (CN) is 12. If the contributions of O(15) and O(16) to the coordination spheres of Cs(1), Cs(3), and Cs(4) are neglected, then for the latter atoms we obtain CN = 16, 14, and 15, respectively. The Cs-O distances, especially those of Cs(3) and Cs(4), show a wide scatter; the average distances are similar for all varieties of cesium atoms (3.49-3.55 ~). Following [2, 3], in the general arrangement of the structure we distinguish ['9V406]O12groups (Fig. 1) of four WO6 octahedra, which are further linked with each other into a three-dimensional framework by sharing vertices. The framework is formed by the six vertices of the W(1)O 6 octahedron and five vertices of each of the W(2)O6, W(3)O6, and W(4)O6 octahedra. The sixth vertex in each of the latter three octahedra may either be part of the coordination polyhedron of Cs(5) or be shared by the W(5)O6 octahedra; the latter form [W406] "tetrahedral" groups, which, together with Cs(5), occupy a wide nonconvex void with the center in the origin (Fig. 2). Extraction of the "statistical"
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TABLE 2. Main Interatomic Distances (/~) in the Structure of Cs2W3Olo
w(1) oetahedron W(1)-O(1) 1.84(2) 1.74(2) o(5) 0(6) 1.91(2) 0(7) 1.92(2) 0(9) 2.17(2) O(10) 1.98(2) Average 1.93
w(2) octahedron 2.06(2) w(2)-o(2) 1.92(2) 0(3) 1.72(2) 0(4) 1.94(2) 0(7) 2.17(2) 0(8) 1.84(2) 0(9) Average 1.94
W(4) octahedron W(4)-O(1)
0(2) 0(6) 0(8) O(11) O(13) Average
2.05(2) 1.85(2) 1.94(2) 1.83(2) 1.70(2) 2.21(2) 1.93
Cs(1) polyhedron Cs(1)-O(1) 3.53(2) x4 0(5) 3.63(2) x4 0(6) 3.19(2) x4 0(11) 3.71(2) x4 0(15) 3.79(27)x2 Average 3.55 Cs(3) polyhedron Cs(3)-o(1) 3.96(2) 0(2) 3.94(2) O(2') 3.45(2) 0(3) 3.17(1) O(4) 3.56(2) O(4') 3.54(2) 0(5) 3.54(2) 0(7) 3.96(2) 0(7') 3.50(2) o(8) 3.21(2) o(9) 3.24(1) o(10) 3.19(2) O(11) 3.47(2) o(12) 3.67(0) o(13) 3.25(2) 0(14) 3.69(2) O(14') 3.64(2) O(16) 2.96(15) Average 3.50
W(3) octahedron 1.92(2) w(3)-o(3) 1.88(2) o(10) o(12) 1.93(1) 1.8o(2) o(13) O(14) 2.1o(2) O(14') 1.80(2) Average 1.91
W(6) octahedron W(5)-O(4) 2.02(2) 0(5) 1.96(2) o(11) 2.00(2) o(15) 1.76(1) O(16) 2.14(18) o(16') 1.73(18) Average 1.93 Cs(2) polyhedron Cs(2)-0(2) 0(3) 0(8) 0(12) O(13) Average
Cs(4) pol ,hedron Cs(4)-o(1) 3.13(2) 3.68(2) o(3) 3.13(2) 0(4) 3.17(1) o(5) 0(6) 3.69(2) 0(6') 3.52(2) 3.11(2) o(7) 3.51(2) o(8) 3.58(2) 0(9) 3.40(2) o(9') O(10) 3.66(2) 3.48(2) o(lo9 O(11) 3.11(2) 3.43(2) 0(13) 3.09(2) O(14) 4.17(10) 0(15) 4.02(19) O(16) O(16') 3.96(18) Average 3.49
3.31(1) x4 3.69(2) x4 3.71(2) x4 3.16(3) x2 3.64(2) x4 3.54
Cs(5) polyhedron Cs(5)-o(4) 3.57(2) x4 3.53(2)• o(5) 3.56(2) x4 O(11) Average 3.55
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Fig. 1. Tetrahedral group [W4018 ] of four WO 6 octahedra (idealized representation).
[Vi/406] groups creates wide channels in the framework, which are parallel to [001] and are occupied by Cs(5) together with Cs(1). The other cesium atoms as well as Cs(1)are located in the intersection regions of other communicating tunnel voids of the framework running along [210], [120], [315], [315], [135], and [L35]. The most thorough treatment of the structure is given in [3]; it explains many of its crystal chemical peculiarities and interprets the structure as a derivative of the AB206 defective pyrochlore structural type [5]. Indeed, a total defective pyrochlore structure would be obtained if all Cs(5) atoms were substituted by the [W406] groups. Then the remaining cesium atoms would have CN = 18, and all vertices of the WO 6 octahedra would be shared, forming a three-dlmensional framework with hexagonal and trigonal loops. The relationship between the tetragonal and cubic cells of the two structures is also rather simple [3]: atetr = acubXX/'lO/2, Ctetr = aeu b. Thus the structure of cesium tritungstate may be regarded as an unusual variant of a substitution-subtraction
B r
Fig. 2. Structure of Cs2W3010 projected along [210]. For simplicity, some details of the figure are omitted. The voids with the [W406] ~ Cs statistical substitution are encircled (only Cs is shown in one circle and only [W406] in another). 648
phase with [x~t406]12+'-'> Cs + substitution in a defective pyrochlore structure. As shown in [3], this substitution should lead to a theoretically nonstoichiometric composition of the polytungstate (complete cell composition is given): Cs20W320108 [Csx(W406)l/3_x/12]. The left part of the formula is an undistorted part of the AB20 6 structure; the formula in square brackets is actually the doubled composition of the void centered by Cs(5). In its full form the latter expression is [Csx(W406)l/3_x/12rn5/3_llx/12]4+ , where rn is a cation vacancy. Proceeding from the fact that the index of each of the terms of this expression is nounegative, we obtain that 0 -< x -< 20/11, from which it follows that the [x~V4016]12+ groups must always be present in the "statistical" voids. On the other hand, since the charges of the particles substituting each other differ greatly, in compounds with a high content of ['~V406] groups the proportion of unoccupied voids increases significantly; this must be energetically unfavorable because of the large size and nonconvexity of the voids. Hence we can conclude that the most preferable value ofx is the value at which the given void is completely occupied by Cs(5) atoms and [W(5)406] groups, i.e., the void with absent vacancies (x = 20/11). Then the composition of the polytungstate will be Cs20W32O108 [Cs20/ll(W406)2/ll] or Cs2W3Olo (Z = 120/11). Hence the occupancy of the W(5) position is 1/11 = 0.0909; this nearly exactly coincides (within lcr) with our values and those obtained in [3]. Consequently, the true composition of the crystal coincides with Cs2W3010 within the experimental error. This indicates that the composition of the phase is nearly constant (stoichiometric), in complete agreement with our data for the Cs2WO4-WO3 system [1]. The same conclusions hold for rubidium tritungstate, which must also be stoichiometric, contrary to the data of [2, 3]. Our results, which indicate the existence of stoichiometric compounds M2W3Olo (M = Rb, Cs) with a fractional number of formula units in the unit cell (Z = 120/11), are rather unusual. Similar examples are available in the literature, but they are few in number; these are mainly examples of highly symmetric structures of stoichiometric compounds in which at least one of the atomic positions may not be occupied by an integer number of atoms of the same variety because of the many-component nature of the compound or inadequate proportions between its components (see, e.g., [6-8]). The compound Kll(V2P3012)6 (Z = 2/3) seems to be closest to our case; in the structure of this compound, which is a derivative of the langbeinite structural type K2Mg2(SO4)3, an unusual substitution K +--- [V30] 7+ is realized. However, even compared to these examples, such a bulky fraction for the number of formula units in the unit cell of M2W3Olo (M = Rb, Cs) is certainly a rarity. This phenomenon may be explained by the high stability of the defective pyrochlore motif for the given combinations of chemical elements; due to this, with certain modifications the structure is realized even for the compositions that are far from the "standard" AB206 composition. This assumption is indirectly confirmed both by the existence of many complex oxides with undistorted defective pyrochlore structures containing tungsten and cesium (rubidium) with additions of some other metals [10-12] and by the presence of a number of pyrochlore-like phases in the Cs-W-O system [2, 13, 14]. Among these phases is CsW206 with an ideal cubic structure of defective pyrochlore [14] as well as layered hexagonal Cs6W11036 [13] and CSs_sW1504s [14], differing from the prototype in subtraction of a part of tungsten-oxygen framework, as in the structure discussed here. Pyrochlore-like phases MxWO3+x/2.yH20 (M = Rb, Cs) may also be obtained from aqueous solutions [15, 16], but their composition and structure are poorly defined because of the lack of reliable single crystal data. The existence of a three-dimensional system of communicating channels in Cs2W3Olo suggests that the compound has ion-exchange and ion-conducting properties, well known for many phases with defective pyrochlore structures [12, 16]. This assumption is justified by the cation-exchange ability of Rb2W3Olo [17], from which metastable pyrochlore-like phases containing H30 +, Li +, Na +, K +, and Ag + were obtained. Possibly, M2W3Olo (M = Rb, Cs) are useful as matrices for the production of intercalation derivatives, which, in turn, may be used as electrochrome materials [15, 16]. REFERENCES .
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Translated by L. Smolina
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