Structural aspects of the ferroelectric, weak-field piezoelectric, and electrostrain responses in Sr, Nb modified Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3

Na0.5Bi0.5TiO3 (NBT)-based materials are widely recognized for their large electrostrain responses (>0.4%). Conventionally, such electrostrain is thought to occur only in an ergodic relaxor state, which is characterized by near-zero weak-field piezoelectric coefficients (d(33)), and a depolarization temperature (T-d) close to or below room temperature. However, our investigation into the (1-x)(Bi-0.5(Na0.8K0.2)(0.5)TiO3]-xSrTi(0.875)Nb(0.1)O(3) (1-x)(NBT-20KBT)-(x)STN) (0.00 <= x <= 0.30) system reveals a similar level of electrostrain (similar to 0.45%) in a composition that exhibits a large weak-field piezoelectric coefficient (d(33)similar to 147 pC/N) and significantly high depolarization temperature (T-d similar to 80 degrees C). Moreover, the system demonstrates atypical features: nonzero d(33) values (240-15 pC/N) over a broad compositional range (x < 0.20) and a coexistence of ferroelectric and ergodic relaxor states across an extended compositional window (0.04 <= x < 0.20). We constructed electric field-temperature (E-T) phase diagrams for all compositions to delineate the thermal stability of the ferroelectric and relaxor states. To elucidate the structural mechanisms underlying field-induced properties, in situ X-ray diffraction experiments under applied electric fields were performed. We interpreted the changes in strain hysteresis behavior across different compositions in terms of field-induced phase transitions, domain switching, and lattice strain processes. Based on these findings, we present a comprehensive composition (x)-electric field (E) phase diagram for the system, offering novel insights into the complex interplay between composition, electric field, and phase transitions in lead-free piezoelectric materials. These results provide a pathway for designing high-performance lead-free piezoelectrics with large electrostrain responses beyond the ergodic relaxor paradigm.