Charles H. Hervoches

Ferroelectric phase transitions in SrBi 2 Nb 2 O 9 and Bi 5 Ti 3 FeO 15 : A powder neutron diffraction study

Structural phase transitions in the Aurivillius phase ferroelectrics SrBi 2 Nb 2 O 9 and Bi 5 Ti 3 FeO 1 5 , containing two and four perovskite layers, respectively, have been studied using powder neutron diffraction. At temperatures below the ferroelectric Curie temperature both materials crystallize in the polar orthorhombic space group A2 1 am. On passing through T C , both phases appear to transform directly to a tetragonal paraelectric phase, space group I4/mmm. This behavior contrasts with that of the analogs Sr 0 . 8 5 Bi 2 . 1 Ta 2 O 9 and SrBi 4 Ti 4 O 1 5 , both of which have been shown unambiguously to transform via an intermediate paraelectric orthorhombic phase, space group Amam.

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

Abstract 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. BiTeO3I and NdTeO3Br both adopt tetragonal symmetry, space group P4/nmm (for BiTeO3I, a=4.10811(8), c=27.988(1) A; NdTeO3Br, a=4.06603(7), c=26.922(1) A, at 25°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 MTe2O5XXMTe2O5XM2O2X, which may be represented by the symbol X13X13X22, 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. NdTeO3Cl adopts an orthorhombically distorted form of this structure type, space group Pmmn, a=4.08096(8), b=4.03441(8), c=25.7582(7) A at 25°C. Bi5TeO8.5I2 adopts a distorted, non-centrosymmetric version of the simpler X13 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.