The crystal structures of BiTeO3I, NdTeO3X (X = Cl, Br) and Bi5TeO8.5I2: some crystal chemistry peculiarities of layered Bi(Ln)-Te oxyhalides

Four new layered oxyhalides related to the Sillen family have been prepared and characterized by Rietveld refinement of powder X-ray and neutron diffr action data. BiTeO3I and NdTeO3Br both adopt tetragonal symmetry, space gro up P4/nmm (for BiTeO3I, a = 4.10811(8), c = 27.988(1) Angstrom; NdTeO3Br, a = 4.06603(7), c = 26.922(1) Angstrom, at 25 degrees C). The structures are composed of triple and double fluorite-related mixed metal oxide layers se parated by single and double halogen layers, in the sequence -MTe2O5-X-X-MT e2O5-X-M2O2-X-, which may be represented by the symbol (X:1X1X22)-X-3-X-3, where the subscript signifies the number of halogen layers and the superscr ipt the number of metal sublayers within the fluorite block, by analogy wit h Sillen's notation. The double fluorite layers are occupied exclusively by Bi, whereas there is an ordered arrangement of Bi/Te within the triple flu orite layers, with Te exclusively occupying the outer sublayers of the bloc k. NdTeO3Cl adopts an orthorhombically distorted form of this structure typ e, space group Pmmn, a = 4.08096(8), b = 4.03441(8), c = 25.7582(7) Angstro m at 25 degrees C. Bi5TeO8.5I2 adopts a distorted, non-centrosymmetric vers ion of the simpler X-:1(3) structure type, space group Cmm2, a = 5.6878(3), b = 5.7230(3), c = 9.7260(6) Angstrom, consisting of single halogen layers sandwiched between triple fluorite layers, in which there is partial order ing of the Bi/Te cations. (C) 2000 Editions scientifiques et medicales Else vier SAS. All rights reserved.