M.Th. Roux

Structure cristalline de la phase β-KEr2F7. Composés isotypes

Abstract Potassium erbium fluoride β-KEr 2 F 7 crystallizes in the orthorhombic system, space group Pna 2 1 , with the unit-cell dimensions a = 11.820, b = 13.333, c = 7.816A ( Z = 8). The crystal structure has been solved from single-crystal diffractometer measurements (Ag Kα ) by Patterson and Fourier syntheses and refined by a least-squares method. The final R value is 0.042 for 2374 independent observed reflections ( R W = 0.051). The four species of erbium atoms are surrounded by eight fluorine atoms. These fluorine atoms form, respectively, three quadratic antiprisms and one dodecahedron, derived from a distorted cube. Two antiprisms and the dodecahedron share two of their faces to form (Er 3 F 17 ) 8− groups. These groups are bidimensionally linked and the planes they form are joined together by the third antiprism. A three-dimensional network is then produced, in the tunnels of which potassium atoms are located. Lattice parameters of compounds which are isotypic to the new structure type of β-KEr 2 F 7 are given.

Synthèse et caractéristiques cristallographiques des phases solides de type fluorine des systèmes PbF2LnF3

Abstract A preliminary study of the PbF2LnF3 systems (Ln = lanthanides and Y) has allowed the characterisation of three phases: a disordered fluorite-like solid solution Pb1−xLnxF2+x the domain of which increases with increasing temperature and dopant ion radius, and two anion-excess fluorite related superstructures: Pb2YF7 (tetragonal, space group I4 or I4 m , a # a F √2 , c # 3a F ) and Pb4Ln3F17 with Ln = SmLu (rhombohedral, space group R3, a h # ( a F √2 )√7, c h # 2a F √3 ). The crystallographic characteristics of the two ordered phases have been confirmed by electron diffraction.

Structure cristalline de CsYb3F10: Compose´s isotypes

Cesium ytterbium fluoride CsYb 3 F 10 crystallizes in the monoclinic system, with parameters a = 4.2893(2) A˚, b = 6.7437(4) A˚, c = 16.196(2) A˚, β = 90°, Z = 2 and space group Pc . The structure has been solved by three-dimensional Patterson and Fourier syntheses and refined by a least-squares method. For 1406 independent reflections recorded on an automatic four-circle diffractometer ( λ AgKα), the final R value is 0.031 ( R W = 0.034). Three edge-sharing pentagonal bipyramids surrounding three ytterbium atoms form Yb 3 F 17 groups and the structure is described by the superposition, according to the sequence AAA … of identical sheets of edge- and corner-sharing Yb 3 F 17 groups. In these sheets, Yb atoms are located at the lattice points of a pseudo-hexagonal network. The nonoccupancy of some of these lattice points creates tunnels, perpendicular to the sheets, in which cesium atoms are located. Lattice parameters of isotypic compounds are given.

Structure cristalline de la phase β-KYb2F7

Abstract β-KYb2F7 crystallizes in the monoclinic system with parameters a = 6.528(1) A, b = 4.217(1) A, c = 6.435(1) A, β = 115°.94, Z = 1, and space group P2. The structure has been solved from single-crystal automatic four-circle diffractometer data by three-dimensional Patterson and Fourier syntheses and refined by a least-squares method. The final R value is 0.040. Structural relations with KLu2F7 are shown. These structures are characterized by chains of edge-sharing semiregular pentagonal bipyramids fused together by corner sharing in two directions perpendicular to the chains. Large tunnels, in which potassium atoms are located, are formed by the stacking of these planes.

Etude des systems CsF-ErF3 et CsF-YbF3

Abstract The phase diagrams of the CsF-ErF 3 and CsF-YbF 3 systems show the existence of the two compounds Cs 3 LnF 6 and CsLn 2 F 7 , like in the other systems CsF-LnF 3 with Ln = Ce, Sm, Eu, Gd, Dy and Ho. But with YbF 3 , we show the existence of three other phases related to α-UO 3 , one of which corresponds with the formula CsYb 3 F 10 . We report the X-ray powder data for all these phases.

Relations structurales entre U3O8 et quelques fluorures mixtes de formules M2M′3F11, MM′2F7 et MM′3F10

Abstract U 3 O 8 oxide, as well as M 2 M ′ 3 F 11 , MM ′ 2 F 7 and MM ′ 3 F 10 fluorides, with M = Rb, Tl, Cs, NH 4 and M ′ = In, Lu, Yb, Tm, is described as the regular repetition according to the … A-A-A … sequence of identical and parallel sheets of edge-and corner-sharing M ′F 7 or UO 7 pentagonal bipyramids and M ′F 6 octahedra. M ′ and U atoms are systematically located at the lattice points of a pseudohexagonal network, but in the fluorides some of these lattice points are vacant, producing hexagonal tunnels in which M atoms are located. It is shown that in the two kinds of compounds the same linear chains and M ′ 3 X 17 groups of pentagonal bipyramids are present, and that the transformation of the U 3 O 8 structure into the fluorides can be achieved by an ordered substitution of some linear … UOUO … chains by … M - M - M … chains. All these structures can be described with the same structural model based on the chemical twinning principle.

Etude par diffraction neutronique de la phase “Cs4−xYb12F40−x” (0 ≤ x ≤ 1): Hypothèse structurale

Abstract The structure of the phase Cs4−xYb12F40−x(0 ≤ x ≤ 1) has been determined by a single-crystal neutron diffraction study. It has been solved in the space group P63mc and refined to the best R factor of 0.0535 for the formula Cs3.4Yb12F39.4 (324 independent reflections). Three edge-sharing pentagonal bipyramids surrounding three ytterbium atoms form Yb3F16 groups and the structure is described as the superposition, according to the sequence A1A2B1B2A1A2…, of sheets of corner-sharing Yb3F16 groups with a possible transformation of bipyramids into octahedra in the A2 and B2 layers. These sheets are joined together by the axial fluorine atoms of the bipyramids or octahedra. Cesium atoms are located in the tunnels formed by their stacking. It is shown that the Cs4−xYb12F40−x phase (0 ≤ x ≤ 1) is an intermediate step of the Cs4−xYb12F40−x solid solution observed with 0 ≤ x ≤ 2 and corresponds to a superstructure of the high-temperature YbF3 phase.