Ultrahigh piezoelectric strain in lead-free BiFeO₃-BaTiO₃ ceramics at elevated temperature

Lead-free Sm-modified 0.67Bi_1.03FeO₃-0.33BaTiO₃ ceramics system was studied systematically and a phase diagram was developed based on crystal structure and electromechanical properties. The Sm³⁺ modification on Bi³⁺-site can change the long-range ferroelectric order of the rhombohedral (R) and tetragonal (T) mixed-phase to the short-range relaxor ferroelectric order with a cubic-like phase. For x = 0.01 composition, simultaneously excellent static and dynamic piezoelectric coefficients (d₃₃ = 275 pC/N and d₃₃* = 350 pm/V) with a high Curie temperature (T_C = 470 °C) were achieved near the morphotropic phase boundary between the R and T phases. Additionally, the room temperature d₃₃* (554 pm/V) for x = 0.03 composition significantly improved to 1030 pm/V at 125 °C. The enhanced piezoelectric strain performance at the crossover boundary of the normal and relaxor ferroelectric is probably related to the local structure heterogeneity due to chemical modification and reversibly phase transition after the removal of the applied electric field. However, the temperature-dependent enhancement in piezoelectric strain originates from thermally activated domain switching. The investigated ceramics system shows a good connection between the crystal structure changes with electromechanical properties. This work provides a new paradigm for designing a new lead-free green energy material for high-temperature piezoelectric applications.