One-step sintering enabled structural stability and defect suppression even in Fe-off-stoichiometric BiFeO3BaTiO3 piezoceramics

BiFeO 3 –BaTiO 3 lead-free piezoelectric ceramics exhibit superior piezoelectric properties while preserving a high Curie temperature. However, given the inherent Gibbs free energy law of BiFeO 3 , the system is difficult to avoid heterogeneous phases such as Bi 25 FeO 39 and/or Bi 2 Fe 4 O 9 , which are accompanied by the volatilization of Bi 3+ and the change of Fe 3+ , 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 0.75BiFeO 3 –0.25BaTiO 3 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 Fe 3+ 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 d 33 reach 132 pC/N and 110 pC/N, respectively. These are significantly higher than those of the most stoichiometric 0.75BiFeO 3 –0.25BaTiO 3 counterparts, setting a new performance record. • Effective suppression of Bi 2 Fe 4 O 9 and Bi 25 FeO 39 heterogeneous phases using one-step low-temperature sintering peocess. • Stabilized piezoelectric properties obtained in both Fe excess and deficiency due to a unique self-regulation mechanism. • Evolutionary behavior of defects with temperature and Fe content. • Confirmed function of oxygen vacancies and Fe 2+ on BiFeO 3 –BaTiO 3 leakage by XPS and First principles calculations.