Study of the pyrochlore-related structure of alpha-Cs_2U_4O_(12) by powder neutron and X-ray diffraction

摘要

The crystal structure of the low temperature alpha variety of the cesium uranate Cs_2U_4O_(12) 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-bar and not R3m as previously determined for the cation framework only) with hexagonal cell parameters a=15.4232(1) A and c=19.1816(2) A. Cs_2U_4O_(12) contains a three-dimensional pyrochlore-like framework of UO_6 octahedra and UO_7 distorted pentagonal bipyramids sharing corners or edges. Bond valence calculations show that, of four U sites, two are only occupied by U~(5+), one by U~(6+) and the fourth by a mixture of U~(6+) and U~(5+). This new structure type derives from the so-called "modified pyrochlore" RbNiCrF_6 type (or CsNbWO_6) by the transformation of some corner-sharing octahedra to edge-sharing pentagonal bipyramids.