Temperature-driven evolution of polar states and phase transitions in 92(Bi_0.5Na_0.5)TiO₃-8BiAlO₃ single crystals near the morphotropic phase boundary

Single crystals of 92Bi_0.5Na_0.5TiO₃-8BiAlO₃ (BNT-8BA) were successfully grown by the flux method. The single crystal exhibits a coexistence of the rhombohedral R3c and pseudocubic Pm—3m phases, with a piezoelectric coefficient d₃₃ of 164 pC/N, a remanent polarization P_r of 25.9 μC/cm², and a coercive field E_c of 41.0 kV/cm, respectively. The electric field-driven irreversible relaxor-to-ferroelectric transition was observed. Notably, in-situ d₃₃ value shows a significant increase in the temperature range of 25 – 82 ℃; meanwhile, the coercive field decreases, the electrostrain is enhanced, and double current peaks appear. Interestingly, the piezoelectric responses exhibit anomalies in the region of 117 − 220 ℃, demonstrating the coexistence of ferroelectric and ergodic relaxor states. The results of the in-situ PFM measurements are consistent with the temperature-dependent variations in piezoelectric and ferroelectric properties. Finally, the transition from rhombohedral to tetragonal phase near 300 ℃ was confirmed via in-situ XRD and PLM analyses.