Study of the pyrochlore-related structure of α-Cs₂U₄O₁₂ by powder neutron and X-ray diffraction

The crystal structure of the low temperature α variety of the cesium uranate Cs₂U₄O₁₂ has been determined by a simultaneous refinement of X-ray and neutron diffraction data obtained on a powder sample. The structure is rhombohedral (space group R3 and not R3m as previously determined for the cation framework only) with hexagonal cell parameters a = 15.4232(1) Å and c = 19.1816(2) Å Cs₂U₄O₁₂ contains a three-dimensional pyrochlore-like framework of UO₆ octahedra and UO₇ distorted pentagonal bipyramids sharing corners or edges. Bond valence calculations show that, of four U sites, two are only occupied by U⁵⁺, one by U⁶⁺ and the fourth by a mixture of U⁶⁺ and U⁵⁺. This new structure type derives from the so-called "modified pyrochlore" RbNiCrF₆ type (or CsNbWO₆) by the transformation of some corner-sharing octahedra to edge-sharing pentagonal bipyramids.