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Abstract: After InTeO3F and InTe2O5F recently described, a new compound In3TeO3F7 is characterized in the In–TeIV–O–F system. The crystal structure was determined by single X-ray diffraction and refined to R 1 =0.028. In3TeO3F7 crystallizes in orthorhombic space group Cmmm, a =7.850(2)Å, b =27.637(6)Å, c =4.098(1)Å, V =889.1(4)Å3 and Z =4. Its structure consists of the stacking, via vertices, of identical layers composed of InF6 and InO2F4 octahedra sharing corners and of InO4F3 pentagonal bipyramids sharing edges and vertices. The Te cations statistically occupy one or the other of two close sites located inside tunnels delimited by the In polyhedra and are bonded to F anions located in the same tunnels. The structure can be considered as an intergrowth of parallel strips of MIn 3 F 10 and hexagonal tungsten bronze (c)-types. It is compared to other structures such as the bronze Sb0.157WO3, TeMo5O16 and Sb2Mo10O31, phases also comprising Te4+ or Sb3+ inside hexagonal tunnels. The electronic lone pair of Te4+ is stereochemically active and a perfect O/F ordering occurs on the anionic sites. [ABSTRACT FROM AUTHOR]

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]

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]