A novel rigid uranyl tungstate sheet in the structures of Na_2[(UO_2)W_2O_8] and alpha- and beta-Ag_2[(UO_2)W_2O_8]

摘要

Crystals of Na_2[(UO_2)W_2O_8] and alpha- and beta-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, P1, a = 6.6484(6), b = 7.5308(7), c = 8.4869(7) A, alpha = 89.952(2), beta = 86.190(2), gamma = 73.299(2) deg, V = 406.02(6) A~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[(UO_2)W_2O_8] (monoclinic, P2_1/c, a = 8.4263(7), b = 7.4897(7), c = 12.927(1) A, beta = 95.443(2) deg, V = 812.2(1) A~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[(UO_2)W_2O_8] (orthorhombic, Pnma, a = 8.6415(6), b = 7.5610(6), c = 12.4513(9) A, V = 813.6(1) A~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 alpha- and beta-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 alpha- and beta-Ag_2 [(UO_2)W_2O_8] are identical within approx 3 percent, 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[(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.