Giant field-induced strain with excellent fatigue performance in (Al_0.5Nb_0.5)⁴⁺-modified 0.85Bi_0.5Na_0.5TiO₃-0.11Bi_0.5K_0.5TiO₃-0.04BaTiO₃ piezoelectric ceramics

Utilizing a conventional solid-state reaction procedure, (Al_0.5Nb_0.5)⁴⁺-modified 0.85Bi_0.5Na_0.5TiO₃-0.11Bi_0.5K_0.5TiO₃-0.04BaTiO₃ (BNKT-BT) samples were synthesized. The impact of (Al_0.5Nb_0.5)⁴⁺ on the crystal structure, ferroelectric, dielectric, and AC impedance properties was systematically investigated. The XRD patterns demonstrate that the crystal structures of all samples exhibit typical pseudo-cubic features. According to the XPS, the doping of (Al_0.5Nb_0.5)⁴⁺ reduces the concentration of oxygen vacancies inside the ceramics, which significantly affects the relaxation properties. This substitution successfully triggers a change from a non-ergodic relaxor (NR) state to an ergodic relaxor (ER) state, which is ascribed to the weakening of the pinning effect of the oxygen vacancies on the polar nanoregions (PNRs). The ceramics exhibit an enormous monopolar strain of 0.468 % under 60 kV/cm, accompanied by an inverse piezoelectric coefficient (d₃₃*) of 780 pm/V, surpassing the performance of numerous other BNT-based piezoelectric ceramics. After 1×10⁴ switching cycles, the strain value changes slightly, indicating excellent fatigue performance. The considerable strain arises from a reversible transformation of the ER phase to the ferroelectric phase induced by an electric field. This research suggests that (Al_0.5Nb_0.5)⁴⁺-doped BNKT-BT ceramics have good prospects for application in actuators and sensors.