Ultra-high piezoelectric properties of BiFeO₃–BaTiO₃ lead-free piezoelectric ceramics enabled by a one-step sintering process

BiFeO₃–BaTiO₃ ceramics have been shown to possess considerable promise in the domain of high-temperature lead-free piezoelectric applications, a property attributable to their elevated Curie temperature and superior piezoelectric properties. However, the inherent issue of high leakage current within this system is a consequence of the unavoidable formation of secondary phases within thermodynamically unstable temperature ranges and the volatility of Bi. This issue severely compromises the high-temperature polarization stability and piezoelectric performance. This high conductance hinders the enhancement of the overall performance and obscures the evolution patterns of intrinsic defects and their influence mechanisms on macroscopic properties, necessitating in-depth investigation. The present study successfully prepared a series of BiFe_1+xO₃–BaTiO₃ ceramics using a one-step sintering process. The Fe content was deliberately designed to exhibit both severe excess and deficiency states. It was found that the Fe non-stoichiometry-induced unique defect evolution behavior. A detailed investigation was conducted to ascertain the influence of the defect configuration on the electrical insulation properties across a range of temperatures and strain levels. The respective contributions of the defect-dipole- and space-charge-induced built-in electric fields to the strain response (before and after polarization) were also examined. It is noteworthy that the pure 0.7BiFeO₃–0.3BaTiO₃ system exhibits remarkable comprehensive properties at room temperature (piezoelectric constant d₃₃^* = 1021 pm/V, strain S ≈ 0.38%, and piezoelectric coefficient d₃₃ = 201 pC/N). The material exhibits a high Curie temperature of approximately 501 °C, accompanied by a notable high-temperature piezoelectric activity. The peak d₃₃ and d₃₃^* values are approximately 380 pC/N (measured at 317.9 °C) and 1481 pm/V (measured at 125 °C), respectively.