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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]

The crystal structure of a new tellurium IV oxyfluoride: KTe3O6F in the series MTe3O6F (M=K, Rb) has been determined on the basis of X-ray single crystal data (rhombohedral symmetry, R-3 space group, a=948.60(10), c=1377.4(2) pm, Z=6, refinement program: Shelxl-97, R1=0.037). The structure is based on twisted Te3O6F sheets, separated by K layers, in which TeO4F square pyramids share O corners and edges, forming hexagonal rings of the same nature as in hexagonal bronzes. The F anions are weakly bonded to Te cations (Te&z.sbnd;F=245.3 pm) and form quasi-independent KF units inserted inside a TeO2 layer structure. The E lone pair is stereochemically active.The O/F order in this phase and in other Te IV oxyfluorides is discussed. [ABSTRACT FROM AUTHOR]