A novel rigid uranyl tungstate sheet in the structures of Na2[(UO2)W2O8] and α- and β-Ag2[(UO2)W2O8]

Crystals of Na-2[(UO2)W2O8] and alpha- and beta-modifications of Ag-2[(UO2)W2O8] were synthesized by high-temperature solid-state reactions and their structures have been determined. The structure of NaAWODW2081 (triclinic, P1, a = 6.6484(6), b = 7.5308(7), c = 8.4869(7) Angstrom, alpha = 89.952(2), beta = 86.190(2), gamma = 73.299(2)degrees, V = 406.02(6) Angstrom(3), Z = 2) was solved by direct methods and refined to R-1 = 0.043 (wR(2) = 0.084). The structure of alpha-Ag-2[(UO2)W2O8] (monoclinic, P2(1)/c, a = 8.4263(7), b = 7.4897(7), c = 12.927(1) Angstrom, = 95.443(2)degrees, V = 812.2(1) Angstrom(3), Z = 4) was solved by direct methods and refined to R-1 = 0.049 (wR(2) = 0.104). The structure of beta-Ag-2 [(UO2)W2O8] (orthorhombic, Pnma, a = 8.6415 (6), b = 7.5610(6), c = 12.4513(g) Angstrom, V = 813.6(l) Angstrom(3), Z = 4) was solved by direct methods and refined to R-1 = 0.059 (wR(2) = 0.154). The crystal structures of the compounds are based upon sheets of UrO(5) pentagonal bipyramids (Ur = UO22+, uranyl ion) and WO6 octahedra with the composition [(UO2)W2O8]. The UrO(5) pentagonal bipyramids share equatorial edges, resulting in chains one polyhedron wide that extend along [010] in all three structures. The WO6 octahedra share comers to form chains of composition [WO5] parallel to [010]. Two adjacent [WO5] chains are linked into double [W2O8] chains by the sharing of edges between WO6 octahedra. The chains of UrO(5) bipyramids and the [W2O8] chains are linked into sheets by the sharing of edges between UrO(5) bipyramids and WO6 octahedra. The sheets are parallel to (100) in Na-2[(UO2)W2O8] and to (001) in alpha- and beta-Ag-2[(UO2)W2O8]. Na and Ag are located in the interlayers, and provide linkage of the sheets. Geometrical parameters of the [(UO2)W2O8] sheets in the structures of Na-2[(UO2)W2O8] and alpha- and beta-Ag-2[(UO2)W2O8] are identical within similar to3%, which indicates a relatively rigid sheet. Thus, the role of the interlayer cations is to compensate the charge of the sheet and to link adjacent sheets. The alpha- and beta-modifications of Ag-2[(UO2)W2O8] differ only in stacking sequences of uranyl tungstate sheets and can be considered as Ag-2[(UO2)W2O8]-2M and Ag-2[(UO2)W2O8]-2O polytypes, respectively. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.