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In the U–Ce–O system, a solid solution (U,Ce)O2+x of fluorite type containing anionic excess is known in a wide composition range. For high values of x, it transforms to a (U1−yCey)4O9−δ phase deriving from the β-U4O9−δ type [ordered anion-excess fluorite superstructure phase; I-43d space group; a=21.7484(1) A˚ for y=0.10]. The crystal structure of (U0.9Ce0.1)4O9−δ has been refined by the Rietveld method on a powder sample measured on D2B at ILL Grenoble. The structural model, proposed by Bevan et al. for β–U4O9−δ and not fully confirmed till now, has been verified. The structure is based on an ordered distribution of cuboctahedral clusters U6O37 inside a fluorite matrix. A preferential ordering of Ce4+ (and U4+) on the so-called “centaur polyhedra” with 10 coordination is proposed, on the basis of bond valence calculations. The structure so determined has the composition M64O143 (MO2.234) and no traces of excess anions, completing the supposed composition up to M4O9, could be detected. [Copyright &y& Elsevier]

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) A˚ and c=19.1816(2) 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 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. [ABSTRACT FROM AUTHOR]