Crystal chemistry of uranyl molybdates. IV. The structures of M-2[(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-radiatio n 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 uniq ue observed (\F-o\ less than or equal to 4 sigma (F)) reflections, respecti vely. The compound Cs-2[(UO2)(6)(MoO4)(7)(H2O)2] is orthorhombic. space gro up Pbcm, a 13.990(2), b 10.808(1), c 25.671(3) Angstrom, V 3881.6(9) Angstr om (3). Z = 4 The compound (NH4)(2)[(UO2)(6)(MoO4)(7)(H2O)(2)] is isostruct ural 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 equ atorial positions of uranyl pentagonal bipyramids that are capped by Ou, at oms. The four symmetrically identical Mo6+ Cations are tetrahedrally coordi nated by O-Ur atoms. The uranyl and molybdate polyhedra link by vertex shar ing to form a framework. with the low-valence M cations located in voids wi thin the framework.