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

Abstract

Abstract A novel hydrated magnesium uranyl germanate, Mg[(UO2)2(Ge2O6(OH)2)]·(H2O)4.4, has been synthesized under hydrothermal conditions at 200 °C. The orthorhombic unit-cell parameters are a=10.829(6), b=7.625(4), c=16.888(10) Å, V=1394.5(1) Å3, space group Cmcm, Z=4. The crystal structure is based on β-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 [Ge2φ8] diortho groups that fill the hexagonal-shaped windows within the sheets. The uranyl germanate layers are connected through Mgφ6 octahedra. The disorder of the [Ge2φ8] 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 β-U3O8 topological-type sheets is provided. Structural complexity parameters (I G,total=176.19 bits/unit cell) indicate that the title compound is one of the simplest actinyl compounds among this family.