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

A novel hydrated magnesium uranyl germanate, Mg[(UO 2 ) 2 (Ge 2 O 6 (OH) 2 )]·(H 2 O) 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) Å 3 , space group Cmcm , Z =4. The crystal structure is based on β-U 3 O 8 -type sheets of corner- and edge-sharing U 6+ O 7 pentagonal bipyramids. The GeO 3 (OH) tetrahedra and GeO 4 (OH) trigonal bipyramids are linked to form [Ge 2 φ 8 ] diortho groups that fill the hexagonal-shaped windows within the sheets. The uranyl germanate layers are connected through Mgφ 6 octahedra. The disorder of the [Ge 2 φ 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 β-U 3 O 8 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.