Phase purity and secondary phase engineering in BNT-based lead-free ceramics via Ti nonstoichiometry

Lead-free Bi_0.5Na_0.5TiO₃-based ceramics have attracted significant research interest due to their promising ferroelectric and piezoelectric properties. However, in practical applications, thermal stability of these properties is equally important. Therefore, the compositions away from the morphotropic phase boundary (MPB) such as 0.85Bi_0.5Na_0.5TiO₃–0.15BaTiO₃ (BNT–15BT), although exhibiting slightly worse piezoelectric properties than the MPB composition like BNT–7BT, are highly favored by manufacturers because of their better thermal stability. However, BNT–xBT solid solutions with high BT content usually show secondary phase after calcination. In this study, Ti non-stoichiometric 0.85Bi_0.5Na_0.5Ti_1−xO₃–0.15BaTi_1−xO₃ (BNBT_1−x) ceramics were prepared. It is found that an appropriate Ti deficiency (x = 0.04) successfully eliminates secondary phases, whereas an even greater Ti deficiency will result in a new secondary phase. Our systematic survey on how the properties, especially depolarization temperature and electrical conductivity, vary with the Ti deficiency emphasizes the cruciality of careful batching procedures to a reliable manufacturing of similar BNT compositions.