Large Electromechanical Response in Lead‐Free La‐Doped BNKT–BST Piezoelectric Ceramics

Piezoceramics 0.99(Bi_0.5Na_0.4K_0.1)_1?xLa_xTiO₃]?0.01Ba_0.7Sr_0.3TiO₃] (BNKT–BST–La_x, x = 0–0.030) were synthesized using a conventional solid‐state reaction method. X‐ray diffraction revealed a phase transition from a tetragonal to cubic phase at x ≥ 0.005. The maximum dielectric constant as well as the depolarization temperature (T_d) decreased with increasing La content. La addition interrupted the polarization and strain hysteresis loops and demonstrates that the ferroelectric order of the BNKT–BST ceramics lead to a reduction in the remnant polarization and coercive field. However, the destabilization of the ferroelectric order is accompanied by a significant increase in the unipolar strain which is highest at x = 0.020 with a value of ~0.39% and corresponding normalized strain, d^*₃₃ (= S_max/E_max) of 650 pm/V. It was observed that the unipolar strain of x = 0.020 is very temperature insensitive up to 125°C, even at 125°C the d^*₃₃ is as high as ~431 pm/V. Moreover, an electric‐field‐dependent XRD was conducted to identify the main source of the high strain and a recoverable transformation from cubic to a rhombohedral–tetragonal mixed phase was observed. The recoverable field‐induced phase transformation is suggested to be the main cause for the obtained large strain at x = 0.020 in the BNKT–BST–La_x ceramics.