Structure/property correlations in the new ferroelectric Bi₅PbTi₃O₁₄Cl and related layered oxyhalide intergrowth phases

The crystal structure and electrophysical properties of the layered intergrowth phase Bi5PbTi3O14Cl have been studied using powder neutron diffraction, ac impedance and second harmonic generation methods. This phase adopts a structure based on a regular intergrowth of Aurivillius-like and Sillen-like blocks in the layer sequence ...[M2O2][M2Ti3O10][M2O2][ Cl]..., and is therefore closely structurally related to the well-known ferroelectric Bi4Ti3O12. The phase adopts the polar orthorhombic space group P2an at temperatures below 590 degreesC, and transforms directly to a centrosymmetric tetragonal phase, space group P4/mmm, at that temperature. This abrupt structural phase transition is consistent with a sudden loss of SHG signal and a dielectric maximum at the same temperature. The structural features and electrophysical behaviour of this phase in relation to Bi4Ti3O12 and related layered oxides/oxyhalides are discussed.