Comprehensive optimization of piezoelectric coefficient and depolarization temperature in Mn-doped Bi_0.5Na_0.5TiO₃-Bi_0.5K_{0. 5}TiO₃-BaTiO₃ lead-free piezoceramics

Lead-free Bi0.5Na0.5TiO3 (BNT) piezoelectric ceramics have the advantages of large coercive fields and high Curie temperatures. But the improvement of piezoelectric coefficient (d(33)) is usually accompanied by a huge sacrifice of depolarization temperature (T-d). In this work, a well-balanced performance of d(33) and T-d is achieved in MnO2-doped 0.79(Bi0.5Na0.5TiO3)-0.14(Bi0.5K0.5TiO3)-0.07BaTiO(3) ternary ceramics. The incorporation of 0.25 mol% MnO2 enhances the d(33) by more than 40%, while T-d remains almost unchanged (i.e., d(33)=181 pC/N, T-d=184 degrees C). X-ray diffraction (XRD) shows that an appropriate fraction of the small axis-ratio ferroelectric phase (pseudo-cubic, P-c) coexists with the long-range ferroelectric phase (tetragonal, T) under this MnO2 doping. Piezoelectric force microscopy (PFM) has revealed a special domain configuration, namely large striped and layered macro domains embedded with small nanodomains. This study provides a distinctive avenue to design BNT-based piezoelectric ceramics with high piezoelectric performance and temperature stability.