The crystal chemistry of uranyl molybdates. II. The crystal structure of iriginite

The structure of synthetic iriginite, [(UO2)Mo2O7(H2O)(2)] (H2O), a 6.705(1), b 12.731(2), c 11.524(2) Angstrom, V 983.6(3) Angstrom (3), Z = 4, has been refined on the basis of F-2 for all unique data collected with monochromatic MoK alpha X-radiation and a CCD-based detector. The agreement factor (R1) was 3.5%, calculated using 532 unique observed reflections (\F-o\ greater than or equal to 4 sigma (F)), and the goodness of-fit (S) was 0.92. This study indicates that iriginite crystallizes in space group Pbcm, rather than Pca2(1) as indicated in a previous investigation. The structure contains one symmetrically distinct U6+ cation that is strongly bonded to two atoms of O, forming a nearly linear uranyl ion (Ur) coordinated by five additional atoms of O arranged at the equatorial corners of pentagonal bipyramids that are capped by the O-Ur atoms. The single symmetrically distinct Mo6+ cation is in a distorted octahedral coordination involving five atoms of O and one H2O group. The structure consists of symmetrically identical sheets of edge-sharing Ur phi (5) pentagonal bipyramids and Mo phi (6) octahedra (phi: unspecified ligand), with composition [(UO2)Mo2O7(H2O)(2)](0). The sheets are linked together via H bonds involving interlayer H2O groups.