Evolutions of dielectric, ferroelectric, and piezoelectric properties with temperature in a nonergodic BNT-BT-KNN single crystal

In this study, <001>_c-oriented 0.93(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-0.01(K_0.5Na_0.5)NbO₃ (BNT-6BT-1KNN) single crystals were successfully synthesized using the flux method, achieving crystal dimensions up to 5 × 5 × 3 mm³. At room temperature, these single crystals exhibit an impressive piezoelectric coefficient (d₃₃) of 264 pC/N, surpassing the performance of equivalent composition in randomly oriented and textured ceramics. Unlike typical (Bi_0.5Na_0.5)TiO₃-based non-ergodic relaxors, the initial strain loop reveals a unique negative minimum strain under a negative electric field, indicating distinct strain characteristics. The application of a poling electric field results in notable changes to both dielectric behaviors and domain structures, signifying a phase transition from polar nanoregions (PNRs) to long-range ferroelectric domains. Temperature-dependent dielectric measurements of the poled specimen reveal an anomaly near the freezing temperature (T_f), indicating a transition between two polar states. Within the temperature range from T_f to 200 °C, dielectric and piezoelectric responses suggest a coexistence of non-ergodic and ergodic relaxor states. Furthermore, a substantial electrostrain of 0.47 % was observed at 175 °C, corresponding to an inverse piezoelectric coefficient (d₃₃^∗) of 940 pm/V, confirming the high electromechanical responsiveness. These findings underscore the potential of BNT-6BT-1KNN single crystals as high-performance lead-free piezoelectric materials for advanced actuator applications.