Dimensional reduction in alkali metal uranyl molybdates:: Synthesis and structure of Cs2[(UO2)O(MoO4)]

A new uranyl molybdate, Cs-2[(UO2)O(MoO4)] (1), has been prepared by high-temperature solid-state reactions. The structure has been solved by direct methods and refined to R-1 = 0.0284 for vertical bar F-0 vertical bar >= 4 sigma(F) The compound 1 is orthorhombic, Pca2(1), a = 12.018(2), b = 12.438(2), c = 17.917(3) angstrom, V = 2678.2(7) angstrom(3), 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 A(x)U(n)Mo(m)O(y) formula (n <= m) as a sum of UMoO6 (= UO2MoO4), A(2)O, and A(2)MoO(4) (A = alkali metal): A(x)U(n)Mo(m)O(y) = kA(2)O + nUMoO(6) + (m-n)A(2)MoO(4), 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 A(2)O and A(2)MoO(4) 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-A(2)O-A(2)MoO(4) compositional diagram. The observed trends of decreasing dimensionality of the uranyl molybdate units allow to specify on this diagram fields of dimensionality.