Ultrahigh Performance in Lead-free Piezoceramics Utilizing a Relaxor Slush Polar State with Multiphase Coexistence

Owing to growing environmental concerns, the development of lead-free piezoelectrics with comparable performance to the benchmark Pb(Zr,Ti)O3 (PZT) becomes of great urgency. However, a further enhancement of lead-free piezoelectrics based on existing strategies has reached a bottleneck. Here we achieve a slush polar state with multiphase coexistence in lead-free potassium-sodium niobate (KNN) piezoceramics, which shows a novel relaxor behaviour, i.e., frequency dispersion at the transition between different ferroelectric phases. It is very different from the conventional relaxor behaviour which occurs at the paraelectric-ferroelectric phase transition. We obtain an ultrahigh piezoelectric coefficient (d33) of 650 ± 20 pC/N, the largest value of non-textured KNN-based ceramics, outperforming that of the commercialized PZT-5H. Atomic-resolution polarization mapping by Z-contrast imaging from different orientations reveals the entire material to comprise polar nanoregions with multiphase coexistence, which i...