Origin of the large strain response in tenary SrTi_0.8Zr_0.2O₃ modified Bi_0.5Na_0.5TiO₃–Bi_0.5K_0.5TiO₃ lead-free piezoceramics

The perovskite oxides (1 − x)Bi_0.5(Na_0.9K_0.1)_0.5TiO₃–xSrTi_0.8Zr_0.2O₃ (SZT1000x, x = 0, 0.2, 0.4, 0.6, 0.8, and 1 %) were prepared via the conventional solid-state reaction method. The room temperature ferroelectric P–E loops coordinate with polarization current density J–E curves illustrated the changes of ferroelectric domains and polar nanoregions under different driving fields exhaustively. The composition and electric field dependent strain behavior of this system were investigated to develop a lead-free piezoelectric material with a large strain response at a lower electric field. A large strain of 0.44 % (S _max/E _max = 744 pm/V) at an applied field of 50 kV/cm was obtained at the composition of 0.6 mol% SZT. Temperature-dependent hysteresis measurements reveal the primary origin of the large strain is due to the presence of a nonpolar phase at a zero field. Upon the application of an electric field, the nonpolar phase that can easily transform into a long-range ferroelectric phase, and then brings the system back to its unpoled state once the applied electric field is removed. Notably, the electric field required to deliver large strains is reduced to 40 kV/cm while the S _max/E _max reached up to 717 pm/V, indicating that the developed material is highly promising for actuator applications.