Structural complexity of barium uranyl arsenates: Synthesis, structure, and topology of Ba4[(UO2)2(As2O 7)3],Ba3[UO2)2(AsO4)2(As207)], and Ba 5Ca[(UO2)8(As04)4O 8]

Three new barium uranyl arsenates, Ba-4[(UO2)(2)(As2O7)(3)] (1), Ba-3[(UO2)(2)(AsO4)(2)(As2O7)] (2), and Ba5Ca[(UO2)(8)(AsO4)(4)O-8] (3), with mono-, pyro-, and mixed arsenate oxoanions have been prepared by high-temperature solid-state reactions. The crystal structures have been solved by direct methods and refined using the least-squares method: 1 - orthorhombic, Pbc2(1), 5.6010(6), b = 13.344(1), c = 31.751(2) angstrom, V = 2373.1(3) angstrom(3), Z = 4, R-1 = 0.044; 2 - monoclinic, C2/c, a = 19.246(2), b = 9.536(1), c = 9.619(1) angstrom, beta = 95.585(6)degrees, V = 1757.0(3) angstrom(3), Z = 4, R-1 = 0.049; 3 - monoclinic, P2(1)/c, a = 12.073(1), b = 12.838(1), c = 13.472(1) angstrom, beta = 116.969(3)degrees, V = 1861.0(3) angstrom(3), Z = 2 R-1 = 0.0584. The compounds demonstrate unprecedented structural topologies (one-dimensional chains in 1, three-dimensional framework in 2, two-dimensional layers in 3), which, however, can be related to previously known structures by topological analysis. The novelty of the compounds obtained can be attributed in part to the synthesis method, which implies the absence of water in the system at the reaction temperature. In turn, the higher ionic strength of divalent cations defines enhanced structural complexity and the deviation of the observed structural topologies from those known for similar systems with monovalent cations