CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. VII. AN IRIGINITE-TYPE SHEET OF POLYHEDRA IN THE STRUCTURE OF [UO2 )MO2O7(H2O)2]

Abstract

A new uranyl molybdate hydrate, [(UO2)Mo2O7(H2O)(2)], has been synthesized by hydrothermal methods. The crystals are dark yellow elongate plates that are invariably twinned on (100). The structure was solved by direct methods and refined on the basis of F-2 for all unique reflections collected with monochromated MoKalpha X-radiation and a CCD (charge-coupled device) detector. The compound is monoclinic, space group C2/c, a 35.071(6), b 6.717(l), c 11.513(2) Angstrom, beta 90.069(6)degrees, V 2712.0(8) Angstrom(3), Z = 12, R1 = 0.055, S = 0.91. The two symmetrically distinct U6+ cations are part Of (UO2)(2+) uranyl ions that are coordinated by five atoms of 0 arranged at the equatorial vertices of pentagonal bipyramids. The three symmetrically independent Mo6+ cations are coordinated by five 0 atoms and one H2O group in distorted octahedral arrangements. The (UO2)O-5 pentagonal bipyramids and MoO5(H2O) octahedra are linked by vertex- and edge-sharing, resulting in electroneutral [(Uo(2))Mo2O7(H2O)(2)](0) sheets that are based upon the iriginite anion-topology. Unlike iriginite, [(UO2)Mo2O7(H2O)(2)](H2O), the structure does not contain H2O groups in the interlayer. The sheets are linked to each other by hydrogen bonding involving H2O groups that are part of the MoO5(H2O) octahedra. [(UO2)Mo2O7(H2O)(2)] is a lower hydrate of iriginite that forms under basic conditions, whereas iriginite usually forms under more acidic conditions.