CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. IV. THE STRUCTURES OF M2[(UO2)6(MO04)7(H2O)2] (M=CS, NH4

Abstract

Two new uranyl molybdate hydrates, occur as transparent yellow needles and yellow elongate plates for M = Cs and NH4, respectively. Their structures were solved by direct methods and refined on the basis of F2 for all unique data collected with MoKα X-radiation and a CCD (charge-coupled device) detector. Final R1 values are 4.1 and 3.9% for M = Cs and M = NH4, respectively, calculated for 2178 and 2969 unique observed (/F0/ ≤ 4σF) reflections, respectively. The compound Cs2[(UO2)6(MoO4)7(H2O)2] is orthorhombic, space group Pbcm, a 13.990(2), b 10.808(1), c 25.671(3) Å, V3881.6(9) Å3, Z = 4. The compound (NH4)2[(UO2)6(MoO4)7(H2O)2] is isostructural with the Cs analogue, a 13.970(1), b 10.747(1), c 25.607(2) Å, V 3844.4(6) Å3, Z = 4. The four symmetrically independent U6+ cations in M2[(UO2)6(MoO4)7(H2O)2] are part of approximately linear (UO2)2+ uranyl ions (Ur) that are further coordinated by five ligands at the equatorial positions of uranyl pentagonal bipyramids that are capped by OUr atoms. The four symmetrically identical Mo6+ cations are tetrahedrally coordinated by O atoms. The uranyl and molybdate polyhedra link by vertex sharing to form a framework, with the low-valence M cations located in voids within the framework.