Mg[(UO2)(2)(Ge2O6(OH)(2)] center dot (H2O)(4.4,) a novel compound with mixed germanium coordination: cation disordering and topological features of beta-U3O8 type sheets

摘要

A novel hydrated magnesium uranyl germanate, Mg[(UO2)(2)(Ge2O6(OH)(2))] center dot (H2O)(4.4), has been synthesized under hydrothermal conditions at 200 degrees C. The orthorhombic unit-cell parameters are a =10.829(6), b = 7.625(4), c= 16.888(10) angstrom, V = 1394.5(1) angstrom(3), space group Cmcm, Z=4. The crystal structure is based on beta-U3O8-type sheets of corner- and edge-sharing U6+O7 pentagonal bipyramids. The GeO3(OH) tetrahedra and GeO4(OH) trigonal bipyramids are linked to form [Ge-2 phi(8)] diortho groups that fill the hexagonal-shaped windows within the sheets. The uranyl germanate layers are connected through Mg phi(6) octahedra. The disorder of the [Ge-2 phi(8)] diortho groups leads to different local structure types with layered- and framework-like characters. A review of the crystal structures of uranyl minerals and actinide-bearing synthetic compounds based on beta-U3O8 topological-type sheets is provided. Structural complexity parameters (I-G,I-total) = 176.19 bits/unit cell) indicate that the title compound is one of the simplest actinyl compounds among this family.