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

The crystal structure of the low temperature a 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 R (3) over bar and not R3m as previously determined for the cation framework only) with hexagonal cell parameters a = 15.4232(l) Angstrom and c = 19. 1816(2) Angstrom. Cs(2)U(4)o(12) 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. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.