Excellent hardening effect in lead-free piezoceramics by embedding local Cu-doped defect dipoles in phase boundary engineering.

Piezoceramics for high-power applications require both high piezoelectric coefficient (d) and mechanical quality factor (Q). However, the trade-off between them poses a significant challenge in achieving high values simultaneously, which is more prominent in lead-free piezoceramics. Here, we propose a new strategy, local Cu-acceptor defect dipoles embedded orthorhombic-tetragonal phase boundary engineering (O-T PBE), to balance d and Q in potassium sodium niobate piezoceramics. This is validated in 0.95(KNa)NbO-0.05(BiNa)HfO-0.2%molFeO-xmol%CuO ceramics. Our strategy simultaneously maintains the O-T PBE and introduces local dimeric and trimeric defects. The dimeric defects form defect dipole polarization that pins domain wall motion, while the trimeric ones introduce the local structural heterogeneity that leads to nano-scale multi-phase coexistence and abundant nano-domains. Encouragingly, for the Cu-doped sample with x = 1, Q increases by a factor of 4, but d only decreases by 1/5 (i.e., achieving a d of 340 pC/N and a Q of 256). Our research provides a new paradigm for balancing d and Q in lead-free piezoceramics, which holds promise for high-power applications.