The crystal structures of BiTeO_3I, NdTeO_3X (X = Cl, Br) and Bi_5TeO_8.5I_2: some crystal chemistry peculiarities of layered Bi(Ln)-Te oxyhalides

摘要

Four new layered oxyhalides related to the Sillen family have been prepared and characterized by Rietveld refinement of powder X-ray and neutron diffraction data. BiTeO_3I and NdTeO3Br both adopt tetragonal symmetry, space group P4mm (for BiTeO_3I, a=4.10811(8), c=27.988(l) A; NdTeO_3Br, a=4.06603(7), c=26.922(1) A, at 25 deg C). The structures are composed of triple and double fluorite-related mixed metal oxide layers separated by single and double halogen layers, in the sequence -Mte_2O_5-X-X-Mte_2O_5-X-M_2O_2-X-, which may be represented by the symbol X_1~3X_1~3X_2~2, where the subscript signifies the number of halogen layers and the superscript the number of metal sublayers within the fluorite block, by analogy with Sillen's notation. The double fluorite layers are occupied exclusively by Bi, whereas there is an ordered arrangement of Bi/Te within the triple fluorite layers, with Te exclusively occupying the outer sublayers of the block. NdTeO_3Cl adopts an orthorhombically distorted form of this structure type, space group Pmmn, a = 4.08096(8), t) = 4.03441(8), C = 25.7582(7) A at 25 deg C. Bi_5TeO_8.5I_2 adopts a distorted, non-centrosymmetric version of the simpler X_1~3 structure type, space group Cmm2, a = 5.6878(3), b = 5.7230(3), C = 9.7260(6) A, consisting of single halogen layers sandwiched between triple fluorite layers, in which there is partial ordering of the Bi/Te cations.