Lead-Free BiFeO₃‑BaTiO₃ Ceramics with High Curie Temperature: Fine Compositional Tuning across the Phase Boundary for High Piezoelectric Charge and Strain Coefficients

BiFeO₃-BaTiO₃ is a promising high-temperature piezoelectric ceramic that possesses both good electromechanical properties and a Curie temperature (T_C). Here, the piezoelectric charge constants (d₃₃) and strain coefficients (d*₃₃) of (1 – x)­BiFeO₃-xBaTiO₃ (BF-xBT; 0.20 ≤ x ≤ 0.50) lead-free piezoelectrics were investigated at room temperature. The results showed a maximum d₃₃ of 225 pC/N in the BF-0.30BT ceramic and a maximum d*₃₃ of 405 pm/V in the BF-0.35BT ceramic, with T_Cs of 503 and 415 °C, respectively. To better understand the performance enhancement mechanisms, a phase diagram was established using the results of XRD, piezoresponse force microscopy, TEM, and electrical property measurements. The superb d₃₃ of the BF-0.30BT ceramic arose because of its location in the optimum point in the morphotropic phase boundary, low oxygen vacancy (V_O^··) concentration, and domain heterogeneity. The superior d*₃₃ of the BF-0.35BT ceramic was attributed to a weak relaxor behavior between coexisting macrodomains and polar nanoregions. The presented strategy provides guidelines for designing high-temperature BF-BT ceramics for different applications.