Coexisting multi-phase and relaxation behavior in high-performance lead-free piezoceramics

Phase boundary engineering (PBE) has remarkably enhanced the piezoelectric properties of potassium sodium niobate {(K, Na)NbO₃, KNN} piezoceramics, yet the physical mechanisms need to be further understood. Here we outline a new physical phenomenon to describe piezoelectricity enhancement in KNN-based ceramics with PBE. We propose that the enhancement is due to the multi-phase coexistence featured with strong relaxation behavior. The strong relaxation behavior was unambiguously revealed by cryogenic experiments and originated from the polar nanoregions (PNRs) exhibiting a scale of 2.1 nm and a weak tetragonality (c/a = 1.0040). in situ temperature-dependent experiments uncovered the thermal evolution of the ferroelectric matrix and PNRs in both unpoled and poled samples, the first report in KNN-based ceramics. Our experiments combined with phenomenological theory revealed that ultra-fine nanodomains, PNRs, and easy polarization rotation together promote macro dielectric and piezoelectric properties in the relaxation-featured multi-phase coexistence. This work reveals the physical mechanism from different levels (e.g., local-mesoscopic-macroscopic), thus providing a new systematic understanding of the observed enhancement of piezoelectricity.