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 (Qm). 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 Qm in potassium sodium niobate piezoceramics. This is validated in 0.95(K0.48Na0.52)NbO3-0.05(Bi0.5Na0.5)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, Qm increases by a factor of 4, but d33 only decreases by 1/5 (i.e., achieving a d33 of 340 pC/N and a Qm of 256). Our research provides a new paradigm for balancing d33 and Qm in lead-free piezoceramics, which holds promise for high-power applications. There is a long-standing trade-off between piezoelectric coefficient and mechanical quality factor for piezoceramics. Here, authors embed local defect dipoles into phase boundary engineering to relieve this relationship in (K, Na) NbO3 piezoceramics.