Structure-property relationships in the lanthanide-substituted PbBi₂Nb₂O₉ Aurivillius phase synthesized by the molten salt method

Samples of PbBi₂Nb₂O₉, PbBi_1.5La_0.5Nb₂O_9, and PbBi_1.5Nd_0.5Nb₂O₉ have been prepared by the molten salt method. The structure, morphology, and electrical properties were investigated. All samples are single-phase and crystallize in an orthorhombic structure with A2₁am symmetry. Neutron diffraction data indicate that the Ln³⁺ cations prefer to occupy the perovskite A-site, whereas Pb/Bi occupy the perovskite A-site and the Bi₂O₂ layer. Changes in unit cell volume are observed on substitution and are attributed to the ionic radii of the Ln³⁺ cations and also correlated to changes in the B-O bond distances in the BO₆ octahedra, which are also observed in IR spectra. SEM images reveal anisotropic plate-like grains, which increase in size with the presence of Ln³⁺ ions. The ferroelectric transition temperature (T_c) decreases with decreasing degree of BO₆ distortion as the influence of the 6s² lone pair of Bi³⁺ is diminished. Relaxor ferroelectric behavior is observed with Ln³⁺ substitution, driven by the disorder of the A-site cations. The room temperature ferroelectric polarization increases with Ln³⁺ substitution, ascribed to the decreased dielectric loss.