HREM studies of complex uranium oxides containing molybdenum and tungsten

HREM, X-ray powder and single-crystal diffraction techniques were used to characterize phases formed in the UO2- MoO2- MoO3- WO3 system by solid state reaction. Some of the phases studied can be described as members of a homologous series of structures built up of ReO3- type slabs of different widths. According to previous X-ray studies, these slabs are connected by U and O atoms so that thin pleated slabs of edge sharing pentagonal UO7- bipyramids are formed. The general formula is UO. MnO3n+1. Structures containing ordered ReO3- type slabs of n =2, 4 and 5 widths have been examined. Alternatively, this set of phases can be regarded as intergrowths of an UVO5- type structure with ReO3- type slabs of different widths. In phases where U is replaced by Mo or W, the ReO3- type slabs are connected by MoO6- octahedra so that six-sided tunnels are formed, in which strings of -O-U-O-U- atoms are found. Hence, an almost planar ordered arrangement of hexagonal UO8- bipyramids and MoO6- octahedra connects the ReO3- type slabs. The general formula of this homologous series of phases is U1-x□ x O.mM O3, where □ = vacancy and m = 2n + 1 (n = number of M O6- octahedra across the ReO3- type slab). The basic network structure can be considered as anintergrowth of slabs of HTB- elements (hexagonal tungsten bronze), one tunnel row wide, with ReO3- type slabs of different widths. Phases built up of ReO3- type slabs with n =3, 4 and 5 with half occupancy of U atoms (x = 0.5) in the six-sided tunnels have been studied, as well as UMo5O16 with full occupancy of all U positions and with ReO3- type slabs two octahedra wide. The U0.5(Mo, W)9 O28 structure contained ordered vacancies. This series is closely related to the intergrowth tungsten bronze (ITB) structures