Layered intergrowth phases Bi4MO8X (X = Cl, M = Ta, and X = Br, M = Ta or Nb):: Structural and electrophysical characterization

The high-temperature structural behavior of the layered inter-growth phase Bi4TaO8Cl, belonging to the Sillen-Aurivillius family, has been studied by powder neutron diffraction. This material is ferroelectric, space group P2(1)cn, at T-C < 640 K. An order-disorder transition to centrosymmetric space group Pmcn is found around 640 K, which involves disordering of TaO6 octahedral tilts. A second phase transition, of a first-order nature, to space group P4/mmm occurs at a temperature of similar to 1038 K. The crystal structures of the bromide analogs Bi4MO8Br (M = Nb, Ta) have also been determined at room temperature; both are isomorphous with Bi4TaO8Cl and exhibit maxima in dielectric constant at temperatures of approximately 588 and 450 K, respectively. (C) 2002 Elsevier Science (USA).