Function design of bismuth layer-structured ferroelectrics

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 P-r and the coercive field E-c 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 P-r, and rare-earth-element substitution at the A site decreased or increased E-c 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.