Function Design of Bismuth Layer-Structured Ferroelectrics

Abstract

Recent research activities of the author's group concerning the structure-property relationship and property design by defect control of bismuth layer-structured ferroelectrics (BLSFs) are described. The polarization measurements for various single-crystal BLSFs showed that the remanent polarization Pr and the coercive field Ec of these ferroelectrics are related to Curie temperature and the number of perovskite units in one layer, m, respectively. For SrBi2Ta2O9 (SBT), Bi substitution at the A site increased Pr, and rare-earth-element substitution at the A site decreased or increased Ec depending on the type and amount of rare-earth element used. These property changes were due to changes in lattice distortion induced by the substitution, and the softening and hardening of polarization property were attained in SBT. A decrease in the concentration of oxygen vacancies was found to be very effective in improving the polarization and insulating properties of Bi4Ti3O12 (BIT). Conductivity analysis and ab initio band-structure calculations showed that the doping of higher-valence cations at the B site and the substitution of rare-earth elements for Bi at the A site decrease the concentration of oxygen vacancies, which cause domain pinning, by charge compensation and lattice stabilization, respectively. Such defect control was demonstrated to be a promising approach to designing the polarization properties of BLSFs.