Defect engineering for control of polarization properties in SrBi ₂Ta₂O₉

Defect engineering to design for remanent polarization (P_r) and coercive field (E_c) in SrBi₂Ta₂O₉ (SBT) is proposed. Cation vacancies coupled with substituents or oxide vacancies are shown to play an essential role in determining the polarization properties. High-resolution neutron powder diffraction studies revealed that trivalent-cation (Bi, La, and Nd) substitution induces Sr vacancies for the requirement of charge neutrality. Polarization measurements of dense ceramics indicated that Bi substitution led to an increase in 2 P_r and the 2P_r value for Sr_0.73Bi_2.18 Ta₂O ₉ was twice as large as that of SBT. Structural refinements revealed that Bi substitution enhances spontaneous polarization (P_s) and the improvement of P_s is responsible for the larger 2P_r observed. La substitution with Sr vacancies increased P_r slightly, but the 2E_c value (41 kV/cm) of La_0.33Sr _0.5Bi₂Ta₂O₉ was much smaller than that for SBT (57 kV/cm). Nd-substituted SBT showed almost the same P_r of La-SBT, while a high 2E_c (125 kV/cm) was attained for Nd _0.33Sr_0.5Bi₂Ta₂O₉. It is suggested that the higher E_c found in Nd-SBT is attributed to oxide vacancies produced by the substitution of a very small amount of Nd at the Ta site. Defect engineering is expected to open the way for the design of the polarization properties in SBT