Crystal chemistry of uranyl molybdates.: IV.: The structures of M2[(UO2)6(MoO4)7(H2O)2] (M = Cs, NH4)

Two new uranyl molybdate hydrates, M-2[(UO2)(6)(MoO4)(7)(H2O)(2)] (M = Cs, NH4), have been synthesized using hydrothermal methods. The crystals 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 F-2 For all unique data collected with MoK alpha 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 (\F-o\ less than or equal to 4 sigma (F)) reflections, respectively. The compound Cs-2[(UO2)(6)(MoO4)(7)(H2O)2] is orthorhombic. space group Pbcm, a 13.990(2), b 10.808(1), c 25.671(3) Angstrom, V 3881.6(9) Angstrom (3). Z = 4 The compound (NH4)(2)[(UO2)(6)(MoO4)(7)(H2O)(2)] is isostructural with the Cs analogue, a 13.970(1), b 10.737(1), c 25.607(2) Angstrom, V 3844,4(6) Angstrom, Z = 4. The four symmetrically independent U6+ cations in M-2[(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 Ou, atoms. The four symmetrically identical Mo6+ Cations are tetrahedrally coordinated by O-Ur 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.