Defect-mediated domain-wall motion and enhanced electric-field-induced strain in hot-pressed K_0.5Na_0.5NbO₃ lead-free piezoelectric ceramics

Since the extrinsic contribution is the key to electric-field-induced strain in ferroelectrics, engineering the interaction between defect and domain-wall motion has been an effective approach for enhancing the strain performance. While acceptor doping has been frequently employed in the lead-free (K, Na)NbO₃ (KNN) system, the individual influence of intrinsic defects is still ill-understood. In this work, pure KNN ceramics with various concentrations of intrinsic defects were prepared by hot-pressing at different temperatures. Meanwhile, the microstructure, electrical properties, and defect chemistry were systematically investigated. An enhanced normalized strain d₃₃* of 320 pm/V with good temperature stability was obtained in the KNN sample hot-pressed at 1000 °C, which is two times larger than that of reported normally sintered KNN. Besides, an asymmetric bipolar strain was found accompanied by the presence of offset polarization. The phenomena can be explained by a qualitative model involving unswitchable domains and intrinsic defects. The present study could enable further understanding of defect engineering and demonstrate a possible manipulation of intrinsic defects to enhance the strain performance of KNN-based piezoceramics.