One-dimensional chains in uranyl tungstates: Syntheses and structures of A8[(UO 2) 4(WO 4) 4(WO 5) 2] ( A=Rb, Cs) and Rb 6[(UO 2) 2O(WO 4) 4

Abstract

Three new uranyl tungstates, A 8[(UO 2) 4(WO 4) 4(WO 5) 2] ( A=Rb ( 1), Cs ( 2)), and Rb 6[(UO 2) 2O(WO 4) 4] ( 3), were prepared by high-temperature solid-state reactions and their structures were solved by direct methods on twinned crystals, refined to R 1 = 0.050 , 0.042, and 0.052 for 1, 2, and 3, respectively. Compounds 1 and 2 are isostructural, monoclinic P2 1/ n, ( 1): a = 11.100 ( 7 ) , b = 13.161 ( 9 ) , c = 25.018 ( 17 ) Å , β = 90.033 ( 13 ) ° , V = 3654 ( 4 ) Å 3 , Z = 8 and ( 2): a = 11.252 ( 2 ) Å , b = 13.815 ( 3 ) Å , c = 25.736 ( 6 ) Å , β = 89.988 ( 2 ) ° , V = 4000.2 ( 16 ) Å 3 , Z = 8 . There are four symmetrically independent U 6+ sites that form linear uranyl [O=U=O] 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 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 2) 4(WO 4) 4(WO 5) 2] 4− chains, consisting of WU 4O 25 pentamers linked by WO 4 tetrahedra and WO 5 square pyramids. The chains run parallel to the a-axis and are arranged in modulated pseudo-2D-layers parallel to (0 1 0). The A + 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 6+ cations is probably responsible for the 1D character of the uranyl tungstate units. The compound 3 is triclinic, P1¯ a = 10.188 ( 2 ) , b = 13.110 ( 2 ) , c = 18.822 ( 3 ) Å , α = 97.853 ( 3 ) , β = 96.573 ( 3 ) , γ = 103.894 ( 3 ) ° , V = 2388.7 ( 6 ) Å 3 , Z = 4 . There are four U positions in the structure with a typical coordination of a pentagonal bipyramid that contain uranyl ions, UO 2 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 2) 2O(WO 4) 4] 6− composed of UO 7 pentagonal bipyramids and W polyhedra. The chains involve dimers of UO 7 pentagonal bipyramids that share common O atoms. The dimers are linked into chains by sharing corners with WO 4 tetrahedra. The chains are parallel to [−101] and are arranged in layers that are parallel to (1 1 1). The Rb + cations provide linkage of the chains into a 3D structure. The compound 1 has many structural and chemical similarities to its molybdate analog, Rb 6[(UO 2) 2O(MoO 4) 4]. However, the compounds are not isostructural. Due to the tendency of the W 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 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.