Polarization switching and rotation in KNN-based lead-free piezoelectric ceramics near the polymorphic phase boundary

(K,Na)NbO3 (KNN)-based ceramics have attracted considerable attention owing to their excellent piezoelectric performance in the polymorphic phase boundary (PPB); however, many researchers have found that the optimal composition usually appears on the tetragonal side near the PPB zone. In this study, it is found that the maximum piezoelectric performance is achieved in the PPB region for unpoled ceramics due to the more efficient and facile polarization switching. However, the most outstanding piezoelectricity shifts to the tetragonal side after the ceramics are poled. Raman spectra and first-principles calculations reveal the occurrence of a phase transformation from a tetragonal to monoclinic structure under an external electric field. Hence, the unpoled tetragonal ceramics transform to a two-phase coexistence condition after the poling process and exhibit the best electrical properties driven by the combined effects of polarization switching and rotation. This study reveals that the electric-field-induced phase transformation leads to the optimal composition on the tetragonal side, and this can provide useful guidance for the design of high-performance KNN-based materials.