Anne-Marie Mercier

Structure determination of β- and γ-BaAlF5 by X-ray and neutron powder diffraction: A model for the α → β γ transitions

{beta}-BaAlF{sub 5} is monoclinic (space group P2{sub 1}/n): a = 5.1517(1) {angstrom}, b = 19.5666(4) {angstrom}, c = 7.5567(2) {angstrom}, {beta} = 92.426(1){degree}, Z = 8. {gamma}-BaAlF{sub 5} is monoclinic (space group P2{sub 1}): a = 5.2584(1) {angstrom}, b = 9.7298(2) {angstrom}, c = 7.3701(2) {angstrom}, {beta} = 90.875(1){degree}, Z = 4. Both structures are determined ab initio from X-ray powder data; final results are given from neutron powder data refinements (R{sub I} = 0.038, R{sub P} = 0.072, and R{sub WP} = 0.087 and R{sub I} = 0.048, R{sub P} = 0.083, and R{sub WP} = 0.101 for the {beta} and {gamma} phases, respectively). Like {alpha}-BaAlF{sub 5}, the {beta} and {gamma} phases are built up from isolated infinite (AlF{sub 5}){sup 2n{minus}}{sub n} chains with AlF{sub 6} octahedra sharing corners in a cis-position. Close structural relationships are shown to exist between the Ba-Al cationic subnetwork of: {alpha}-BaAlF{sub 5} and the CrB-type structure; {beta}-BaAlF{sub 5} and the SrAg-type; {gamma}-BaAlF{sub 5} and the FeB-type.

Synthesis and crystal structure of γ-BaZrF6

[gamma]-BaZrF[sub 6] is obtained from hydrothermal synthesis in HF solution. It crystallizes in the monoclinic system, with space group C2/c; a = 13.193(2), b = 7.499(1), and c = 19.839(3) [Angstrom]; [beta] = 91.69(1)[degree]; and Z = 16. The structure is determined from X-ray single crystal data, R = 0.016 for 3390 unique observed reflections. Barium atoms are inserted between isolated [ZrF[sub 6]][sup 2n[minus]][sub n] chains, built up from ZrF[sub 8] bicapped trigonal prisms, linked alternately by faces and opposite vertices. A site has been found partly occupied by probably a water molecule, leading to the estimated BaZrF[sub 6](H[sub 2]O)[sub 0.04] formula for the studied crystal.

Distorted chiolite crystal structures of α-Na5M3F14 (M=Cr,Fe,Ga) studied by X-ray powder diffraction

The crystal structures of the chiolite-related room temperature phases α-Na 5 M 3 F 14 (M III =Cr,Fe,Ga) are determined. For all of them, the space group is P 2 1 / n , Z =2; a =10.5096(3) A, b =7.2253(2) A, c =7.2713(2) A, β=90.6753(7)° (M=Cr); a =10.4342(7) A, b =7.3418(6) A, c =7.4023(6) A, β=90.799(5)° (M=Fe), and a =10.4052(1) A, b =7.2251(1) A, c =7.2689(1), β=90.6640(4)° (M=Ga). Rietveld refinements produce final R F factors 0.036, 0.033, and 0.035, and R WP factors, 0.125, 0.116, and 0.096, for M III =Cr, Fe, and Ga, respectively. The MF 6 polyhedra in the defective isolated perovskite-like layers deviate very few from perfect octahedra. Subtle octahedra tiltings lead to the symmetry decrease from the P 4/ mnc space group adopted by the Na 5 Al 3 F 14 chiolite aristotype to the P 2 1 / n space group adopted by the title series. Facile twinning precluded till now the precise characterization of these compounds.

Sr5(VIVOF5)3F(H2O)3 refined from a non-merohedrally twinned crystal

The title compound, pentastrontium tris[pentafluoridooxidovanadate(IV)] fluoride trihydrate, was obtained under hydrothermal conditions. Its crystal structure has been refined from intensity data of a non-merohedrally twinned crystal. Two domains in almost equal proportions are related by a −180° rotation along the reciprocal [101]* vector. The structure may be considered as a derivative of the fluorite structure type, adopted here by SrF2. In the title compound, fluorite-like large rods are recognized, built up from a group of 16 Sr atoms of which 6 are substituted by V atoms, leading to [Sr10V6]∞ units. These rods extend infinitely along the b axis and are interconnected by the three water molecules. Each of the water molecules is shared by two different Sr atoms belonging to two different rods. The rods are also interconnected by an ‘independent’ F atom in a distorted triangular [FSr3] coordination and by hydrogen-bonding interactions via donor water molecules. The acceptors are either F atoms or the O atoms of the vanadyl ion, VO2+, that is part of the [VOF5] isolated octahedron.

[Cu2(HF2)(H2O)8][FeF6]·2H2O

The title compound, octaaqua(hydrogenfluorido)dicopper(II) hexafluoridoferrate(III) dihydrate, was synthesized under hydrothermal conditions. The Cu atom is coordinated by one F and five O atoms within a highly distorted octahedron, forming dimeric [Cu2(H2O)8HF2]3+ units by edge sharing. These units are hydrogen bonded to [FeF6]3− anions and to an interstitial water molecule. The former feature Fe3+ on a special position (). The dimeric copper units are linked to adjacent dimers by F—H⋯F hydrogen bonds. Additional O—H⋯O and O—H⋯F hydrogen bonds help to consolidate the crystal packing.