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 intergrowth phase Bi4TaO8Cl, belonging to the Sillén-Aurivillius family, has been studied by powder neutron diffraction. This material is ferroelectric, space group P21cn, at TC<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 ~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.