Oxygen-Vacancy-Controlled Abnormal Thermal Depolarization in Bismuth Ferrite–Barium Titanate Ceramics

Although bismuth ferrite–barium titanate (BF–BT) ceramics are considered for high-temperature applications due to their ultrahigh Curie temperature ( T C ), an understanding of their piezoelectric behavior under high temperature remains insufficient. Unlike conventional lead-based piezoelectrics with a thermal depolarization temperature ( T d ) slightly lower than T C, the piezoelectric coefficient ( d 33 ) of BF–BT decreases rapidly far below T C . Combined with in situ structure elucidation and theoretical calculation, we demonstrate that the vanish of d 33 is not from genuine polarization degradation but is closely related to oxygen vacancies. That is, the high concentration and diffusivity of oxygen vacancies at elevated temperatures are primarily responsible for the sharp increase in electrical conductivity, leading to an internal discharging effect and then an “apparent loss” of d 33 .