Layered Intergrowth Phases Bi4MO8X (X=Cl, M=Ta and X=Br, M=Ta or Nb): Structural and Electrophysical Characterization

Abstract The high-temperature structural behavior of the layered intergrowth phase Bi 4 TaO 8 Cl, belonging to the Sillen-Aurivillius family, has been studied by powder neutron diffraction. This material is ferroelectric, space group P 2 1 cn , at T C Pmcn is found around 640 K, which involves disordering of TaO 6 octahedral tilts. A second phase transition, of a first-order nature, to space group P 4/ mmm occurs at a temperature of ∼1038 K. The crystal structures of the bromide analogs Bi 4 M O 8 Br ( M =Nb, Ta) have also been determined at room temperature; both are isomorphous with Bi 4 TaO 8 Cl and exhibit maxima in dielectric constant at temperatures of approximately 588 and 450 K, respectively.