S. Aléonard

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.

Etude du system KF-ErF3

Abstract The phase diagram of the KF-ErF 3 system shows the existence of the five compounds K 3 ErF 6 , K 2 ErF 5 , KErF 4 , KEr 2 F 7 and KEr 3 F 10 . We give their X-ray single crystal and powder data (except for K 2 ErF 5 ). • - The monoclinic phase K 3 ErF 6 is closely related to the perovskite structure and • - The three others are closely related to the fluorite-type structure.

Etude du systeme RbFErF3

Abstract The phase diagram of the RbF-ErF3 system shows the existence of the four compounds Rb3ErF6, Rb2ErF5, RbEr3F10 and RbEr2F7. We report the X-ray powder data for the three first ones and X-ray single crystal data for RbEr2F7.

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 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.

Structure cristalline de Pb8Y6F32O

Abstract Pb8Y6F32O crystallizes with trigonal rhombohedral symmetry (space group R 3 ). The hexagonal unit cell parameters are a = 10,817(2), c = 19,942(5) A, Z = 3. The crystal structure has been determined from X-ray data collected on a single-crystal diffractometer (AgKα radiation). The structure has been solved by Patterson and Fourier syntheses and refined by a least-squares method. The final R value is 0.059 for 1679 independent observed reflections. The structure is strongly fluorite related. It is characterized by the presence of Y6F36 clusters which occupy plane hexagonal lattice points, the stacking of which, along the c direction, in the sequence ABCABC, forms a three-dimensional framework of Y6F36 clusters within a fluorite-type matrix built by the lead polyhedra. Y6F36 clusters consist of an octahedral arrangement of six YF8 square antiprisms sharing corners to enclose a cuboctahedron of fluorine ions. Polyhedra formed by the two species of lead atoms derive from distorted cubes. Polyhedra surrounding Pb(2) atoms form (Pb6F32) blocks enclosing a cube of fluorine atoms with the oxygen anion at its center.

Structure cristalline de la phase β-KEr3F10

Abstract Potassium erbium fluoride β-KEr3F10 crystallizes in the monoclinic system, space group Cm, with the unit cell dimensions a = 14.0888 A, b = 8.1378 A, c = 28.280A β = 109°27′ (Z = 16). Single-crystal diffraction data collected on an automatic four-circle diffractometer have been refined by full-matrix least-squares calculations to a weighted R′ value of 0.083 (unweighted R = 0.090) for 2800 independent observed reflections. Just as KY3F10 and β-KYb3F10, the structure of β-KEr3F10 is characterized by the presence of (Ln6F32)14− blocks. These groups correspond to the whole “vacant anionic cube surrounded by six filled anionic cubes,” characteristic of the fluorite structure, with a rotation of 45° in their plane of the external faces of the filled cubes. Just as the two other compounds, these blocks are two dimensionally linked to form layers. Their complex sequence is compared to that observed in α-KEr3F10.

Structure cristalline de K0,265Gd0,735F2,47

K0.265Gd0.735F2.47, the extreme member (2ac = 0.47) of the series KO.5-JCG<1O.5+XF2+2X of ordercd anion excess fluorite-related structures crystallizes in the orthorhombic space group Immm, with the xmit cell dimensions: a = 11.652(4) A, 6 = 8.152(2) Ä, c = 32.57(1) Ä, Z = 64. Structure analysis shows the presence of Gd6F37 groups. They consist of an arrangement of six GdFg Square antiprisms sharing comers which Surround a cuboctahedron of anions with an additional anion at its center. The GdeFsv groups are distributed in an ordered manner within a fluoritetype matrix and form chains parallel to the b direction. These chains altemate with edge-shared GdFg cubes chains to form layers running perpendicular to the a direction. The layers are connected by the distorted polyhedra surrounding potassium and the mixed-cation [Ko.62Gdo.38]-

Phases de structure du type fluorine dans le systéme KFErF3

Resume Les trois composes KErF 4 , KEr 2 F 7 , et KEr 3 F 10 sont etroitement relies a la structure fluorine. Nous reproduisons leurs reseaux reciproques et proposons des modeles de structure pour ces trois composes.

Chemical preparation and X-ray structure determination of K0.30NbF3

Abstract The preparation and crystal structure of K 0.30 NbF 3 single crystals are described. The compound is orthorhombic with a = 7.540(3), b = 13.06(2), c = 7.750(3), A, space group C 222 1 . The structure refines to R = 0.044 with 455 reflections. The framework derives from the Magneli hexagonal tungsten bronze and is similar to the K 0.25 VF 3 orthorhombic phase.