Crystal chemistry of uranyl molybdates. VI. New uranyl molybdate units in the structures of Cs4[(UO2)3O(MoO4)2(MoO5)] and Cs6[(UO2)(MoO4)4]

Abstract

Two Cs uranyl molybdates, Cs4((UO2)3O(MoO4)2(MoO5)) and Cs6((UO2)(MoO4)4), have been synthesized by high- temperature solid-state reactions. The structures of these compounds were solved by direct methods and refined on the basis of F2 for all unique data collected with monochromatic MoK X-radiation and a CCD (charge-coupled device) detector. The structure of Cs4((UO2)3O(MoO4)2(MoO5)) was refined to an agreement factor ( R1) of 4.4%, calculated using the 4873 unique observed reflections (Fo ≥ 4F). It is triclinic, space group P1, a 7.510(2), b 7.897(2), c 9.774(2) Å, 79.279(5), 81.269(5), 87.251(5)°, V 562.8(2) Å3, Z = 1. The structure of Cs 6((UO2)(MoO4)4) was refined to an R1 of 4.9%, calculated using the 4275 unique observed reflections (Fo ≥ 4F). It is triclinic, space group P¯ 1, a 11.613(3), b 12.545(3), c 14.466(3) Å, 102.713(6), 95.281(6), 106.182(6) o, V 1947.7(8) Å3, Z = 3. These compounds are based upon uranyl molybdate structural units not previously observed in uranyl compounds. The structure of Cs 4((UO2)3O(MoO4)2(MoO5)) contains sheets of composition ((UO 2)3Mo3O14)4- that contain UrO5 pentagonal bipyramids (Ur: uranyl ion), MoO4 tetrahedra and MoO5 polyhedra. The sheets are parallel to (100) and Cs cations are located in the interlayer. The structure of Cs 6((UO2)(MoO4)4) is based upon two symmetrically distinct finite clusters of composition ((UO2)(MoO4)4)6-, each of which contains a central UrO4 square bipyramid that shares all four of its equatorial vertices with MoO4 tetrahedra. Three-dimensional connectivity is provided by Cs cations located between the clusters.