Tubular Chains, Single Layers, and Multiple Chains in Uranyl Silicates: A2[(UO2)Si4O10] (A = Na, K, Rb, Cs)

Abstract

Four new uranyl silicates with the same framework composition but different crystal structures, Na2[(UO2)Si4O10]·0.5H2O (1), K2[(UO2)Si4O10] (2), Rb2[(UO2)Si4O10] (3), and Cs2[(UO2)Si4O10] (4), have been synthesized under supercritical hydrothermal conditions at 590 °C and 1620 bar and characterized by single-crystal X-ray diffraction and infrared and photoluminescence spectroscopy. Crystal data for compound 1: tetragonal, I4/mcm (No. 140), a = 18.0332(7) Å, c = 7.7752(5) Å, V = 2528.5(2) Å3, Z = 8, R1 = 0.0447; compound 2: monoclinic, P21/n (No. 14), a = 8.2970(17) Å, b = 7.7403(15) Å, c = 8.7906(18) Å, β = 104.83(3)°, V = 545.74(19) Å3, Z = 2, R1 = 0.0140; compound 3: monoclinic, P21/c (No. 14), a = 10.3483(3) Å, b = 23.7297(6) Å, c = 7.6278(2) Å, β = 110.482(1)°, V = 1754.68(8) Å3, Z = 6, R1 = 0.0194; compound 4: orthorhombic, Cmca (No. 64), a = 7.7157(3) Å, b = 19.8719(8) Å, c = 24.0160(10) Å, V = 3682.3(3) Å3, Z = 12, R1 = 0.0277. The silicate anions in the structures of these compounds form tubular chains, single layers, and multiple chains which are linked by UO6 tetragonal bipyramids to form three-dimensional frameworks. The tubular chains and single layers are of the new type with respect to the sequence of vertex orientations. The multiple chain is observed for the first time and contains Q2, Q3, and Q4 Si. The structures of these silicate anions can be correlated with the sizes of alkali metal cations.