A novel rigid uranyl tungstate sheet in the structures of Na 2[(UO 2)W 2O 8] and α- and β-Ag 2[(UO 2)W 2O 8

Abstract

Crystals of Na 2[(UO 2)W 2O 8] and α- and β-modifications of Ag 2[(UO 2)W 2O 8] were synthesized by high-temperature solid-state reactions and their structures have been determined. The structure of Na 2[(UO 2)W 2O 8] (triclinic, P 1 , a=6.6484(6), b=7.5308(7), c=8.4869(7) Å, α=89.952(2), β=86.190(2), γ=73.299(2)°, V=406.02(6) Å 3, Z=2) was solved by direct methods and refined to R 1=0.043 ( wR 2=0.084). The structure of α-Ag 2[(UO 2)W 2O 8] (monoclinic, P2 1/ c, a=8.4263(7), b=7.4897(7), c=12.927(1) Å, β=95.443(2)°, V=812.2(1) Å 3, Z=4) was solved by direct methods and refined to R 1=0.049 ( wR 2=0.104). The structure of β-Ag 2[(UO 2)W 2O 8] (orthorhombic, Pnma, a=8.6415(6), b=7.5610(6), c=12.4513(9) Å, V=813.6(1) Å 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 = UO 2 2+, uranyl ion) and WO 6 octahedra with the composition [(UO 2)W 2O 8]. The UrO 5 pentagonal bipyramids share equatorial edges, resulting in chains one polyhedron wide that extend along [010] in all three structures. The WO 6 octahedra share corners to form chains of composition [WO 5] parallel to [010]. Two adjacent [WO 5] chains are linked into double [W 2O 8] chains by the sharing of edges between WO 6 octahedra. The chains of UrO 5 bipyramids and the [W 2O 8] chains are linked into sheets by the sharing of edges between UrO 5 bipyramids and WO 6 octahedra. The sheets are parallel to (100) in Na 2[(UO 2)W 2O 8] and to (001) in α- and β-Ag 2[(UO 2)W 2O 8]. Na and Ag are located in the interlayers, and provide linkage of the sheets. Geometrical parameters of the [(UO 2)W 2O 8] sheets in the structures of Na 2[(UO 2)W 2O 8] and α- and β-Ag 2[(UO 2)W 2O 8] are identical within ∼3%, 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 α- and β-modifications of Ag 2[(UO 2)W 2O 8] differ only in stacking sequences of uranyl tungstate sheets and can be considered as Ag 2[(UO 2)W 2O 8]-2M and Ag 2[(UO 2)W 2O 8]-2O polytypes, respectively.