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 (d₃₃) and mechanical quality factor (Q_m). 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 d₃₃ and Q_m in potassium sodium niobate piezoceramics. This is validated in 0.95(K_0.48Na_0.52)NbO₃-0.05(Bi_0.5Na_0.5)HfO₃-0.2%molFe₂O₃-xmol%CuO ceramics. Our strategy simultaneously maintains the O-T PBE and introduces local dimeric \({({{Cu}}_{{Nb}}^{{\prime} {\prime} {\prime} }-{V}_{O}^{\bullet \bullet })}^{{\prime} }\) and trimeric \({\left({V}_{O}^{\bullet \bullet }-{{Cu}}_{{Nb}}^{{\prime} {\prime} {\prime} }-{V}_{O}^{\bullet \bullet }\right)}^{\bullet }\) 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_m increases by a factor of 4, but d₃₃ only decreases by 1/5 (i.e., achieving a d₃₃ of 340 pC/N and a Q_m of 256). Our research provides a new paradigm for balancing d₃₃ and Q_m in lead-free piezoceramics, which holds promise for high-power applications.