One-dimensional chains in uranyl tungstates: Syntheses and structures of A{sub 8}[(UO{sub 2}){sub 4}(WO{sub 4}){sub 4}(WO{sub 5}){sub 2}] (A=Rb, Cs) and Rb{sub 6}[(UO{sub 2}){sub 2}O(WO{sub 4}){sub 4}]
- Nizhny Novgorod State University, 603095 Nizhny Novgorod (Russian Federation)
- Department of Crystallography, Federal Agency of Science and Education, St. Petersburg State University, RU-199034, St. Petersburg (Russian Federation)
- Institut fuer Geowissenschaften, Universitaet Kiel, D-24118 Kiel (Germany)
- Laboratory of Chemical and Mineralogical Crystallography, University of Bern, CH-3012 Bern (Switzerland)
Three new uranyl tungstates, A{sub 8}[(UO{sub 2}){sub 4}(WO{sub 4}){sub 4}(WO{sub 5}){sub 2}] (A=Rb (1), Cs (2)), and Rb{sub 6}[(UO{sub 2}){sub 2}O(WO{sub 4}){sub 4}] (3), were prepared by high-temperature solid-state reactions and their structures were solved by direct methods on twinned crystals, refined to R{sub 1}=0.050, 0.042, and 0.052 for 1, 2, and 3, respectively. Compounds 1 and 2 are isostructural, monoclinic P2{sub 1}/n (1): a=11.100(7), b=13.161(9), c=25.018(17)A, {beta}=90.033(13){sup o}, V=3654(4)A{sup 3}, Z=8 and (2): a=11.252(2)A, b=13.815(3)A, c=25.736(6)A, {beta}=89.988(2){sup o}, V=4000.2(16)A{sup 3}, Z=8. There are four symmetrically independent U{sup 6+} sites that form linear uranyl [O=U=O]{sup 2+} cations with rather distorted coordination in their equatorial planes. There are six W positions: W(1) and W(2) have square-pyramidal coordination (WO{sub 5}), whereas W(3), W(4), W(5), and W(6) are tetrahedrally coordinated. The structures are based upon a novel type of one-dimensional (1D) [(UO{sub 2}){sub 4}(WO{sub 4}){sub 4}(WO{sub 5}){sub 2}]{sup 4-} chains, consisting of WU{sub 4}O{sub 25} pentamers linked by WO{sub 4} tetrahedra and WO{sub 5} square pyramids. The chains run parallel to the a-axis and are arranged in modulated pseudo-2D-layers parallel to (010). The A{sup +} cations are in the interlayer space between adjacent pseudo-layers and provide a 3D integrity of the structures. Compounds 1 and 2 are the first uranyl tungstates with 2/3 of W atoms in tetrahedral coordination. Such a high concentration of low-coordinated W{sup 6+} cations is probably responsible for the 1D character of the uranyl tungstate units. The compound 3 is triclinic, P1-bar a=10.188(2), b=13.110(2), c=18.822(3)A, {alpha}=97.853(3), {beta}=96.573(3), {gamma}=103.894(3){sup o}, V=2388.7(6)A{sup 3}, Z=4. There are four U positions in the structure with a typical coordination of a pentagonal bipyramid that contain uranyl ions, UO{sub 2}{sup 2+}, as apical axes. Among eight W sites, the W(1), W(2), W(3), W(4), W(5), and W(6) atoms are tetrahedrally coordinated, whereas the W(7) and W(8) cations have distorted fivefold coordination. The structure contains chains of composition [(UO{sub 2}){sub 2}O(WO{sub 4}){sub 4}]{sup 6-} composed of UO{sub 7} pentagonal bipyramids and W polyhedra. The chains involve dimers of UO{sub 7} pentagonal bipyramids that share common O atoms. The dimers are linked into chains by sharing corners with WO{sub 4} tetrahedra. The chains are parallel to [-101] and are arranged in layers that are parallel to (111). The Rb{sup +} cations provide linkage of the chains into a 3D structure. The compound 1 has many structural and chemical similarities to its molybdate analog, Rb{sub 6}[(UO{sub 2}){sub 2}O(MoO{sub 4}){sub 4}]. However, the compounds are not isostructural. Due to the tendency of the W{sup 6+} cations to have higher-than-fourfold coordination, part of the W sites adopt distorted fivefold coordination, whereas all Mo atoms in the Mo compound are tetrahedrally coordinated. Distribution of the WO{sub 5} configurations along the chain extension does not conform to its 'typical' periodicity. As a result, both the chain identity period and the unit-cell volume are doubled in comparison to the Mo analog, which leads to a new structure type.
- OSTI ID:
- 20902463
- Journal Information:
- Journal of Solid State Chemistry, Vol. 179, Issue 10; Other Information: DOI: 10.1016/j.jssc.2006.05.015; PII: S0022-4596(06)00291-X; Copyright (c) 2006 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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