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

The crystal structure of the low temperature <f>α</f> 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 <f>R<ovl type="bar" STYLE="S">3</ovl></f> and not <f>R3m</f> as previously determined for the cation framework only) with hexagonal cell parameters <f>a=15.4232(1)</f> A˚ and <f>c=19.1816(2)</f> A˚. 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 U<f>5+</f>, one by U<f>6+</f> and the fourth by a mixture of U<f>6+</f> and U<f>5+</f>. 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. [ABSTRACT FROM AUTHOR]