Dimensional Reduction in Alkali Metal Uranyl Molybdates: Synthesis and Structure of Cs2[(UO2)O(MoO4)

Abstract

AbstractA new uranyl molybdate, Cs2[(UO2)O(MoO4)] (1), has been prepared by high‐temperature solid‐state reactions. The structure has been solved by direct methods and refined to R1 = 0.0284 for |F0| ≥4σF. The compound 1 is orthorhombic, Pca21, a = 12.018(2), b = 12.438(2), c = 17.917(3) Å, V = 2678.2(7) Å3, Z = 12. The structure of 1 is based upon one‐dimensional chains consisting of corner‐sharing UO6 and MoO4 polyhedra. The UO6 square bipyramids share cis equatorial vertices to form zigzag chains that are further stabilized by bidendate MoO4 tetrahedra. The chains have composition [(UO2)O(MoO4)]2− and are oriented parallel to the c axis. In the structure, adjacent chains form pseudo‐2D‐layers parallel to (010). The Cs+ cations are in between the pseudolayers. The composition‐structure relationships in alkali metal uranyl molybdates can be rationalized from the viewpoint of dimensional reduction using the following assumptions: (1) representation of the AxUnMomOy formula (n ≤ m) as a sum of UMoO6 (= UO2MoO4), A2O, and A2MoO4 (A = alkali metal): AxUnMomOy = kA2O + nUMoO6 + (m‐n)A2MoO4, where x = 2(k + m − n) and y = k + 2n + 4m; (2) analysis of the dimensionality of uranyl molybdate structural units and its changes upon incorporation of ionic reagents A2O and A2MoO4 into a parent framework of UO2MoO4 that consists of 3‐D array of U and Mo based polyhedra. The relationships between different compositions and structures may be visualized using the UO2MoO4–A2O–A2MoO4 compositional diagram. The observed trends of decreasing dimensionality of the uranyl molybdate units allow to specify on this diagram fields of dimensionality.