One-step sintering enabled structural stability and defect suppression even in Fe-off-stoichiometric BiFeO₃—BaTiO₃ piezoceramics

BiFeO₃—BaTiO₃ lead-free piezoelectric ceramics exhibit superior piezoelectric properties while preserving a high Curie temperature. However, given the inherent Gibbs free energy law of BiFeO₃, the system is difficult to avoid heterogeneous phases such as Bi₂₅FeO₃₉ and/or Bi₂Fe₄O₉, which are accompanied by the volatilization of Bi³⁺ and the change of Fe³⁺, resulting in low insulating properties and high dielectric loss. These factors hinder the enhancement of polarizability and the overall performance at elevated temperatures and electric field conditions. The present study focuses on a highly leaky 0.75BiFeO₃–0.25BaTiO₃ ceramic, in which the Fe content is deliberately designed to be both severely excessive and deficient, and is prepared using a one-step low-temperature sintering process. It is noteworthy that the structural stability and defect suppression, even in this challenging system, are achieved via the one-step low-temperature sintering. This samples exhibit a distinctive self-tuning property and an excellent stability over a wide compositional range. First-principles density functional theory calculations and XPS analysis have for the first time confirmed that suppressing oxygen vacancies and Fe³⁺ valence states can reduce the concentration and mobility of hole carriers, thereby effectively reducing leakage current, with the mechanism shifting from ohmic conduction to space-charge-limited conduction. Even under the extreme compositional conditions of x = ± 5 and a low sintering temperature, the piezoelectric coefficients d₃₃ reach 132 pC/N and 110 pC/N, respectively. These are significantly higher than those of the most stoichiometric 0.75BiFeO₃–0.25BaTiO₃ counterparts, setting a new performance record.