Gradient-Acceptor-Doping Breaks the Piezoelectric Trade-Off in Lead-Free (K, Na)NbO₃ Ceramics

Achieving a high piezoelectric coefficient (d₃₃) and a high mechanical quality factor (Q_m) simultaneously in ferroelectric ceramics has been a persistent challenge, as conventional doping strategies, i.e., donor doping and acceptor doping, inevitably sacrifice one for the other. Here, the inherent trade-off is breaking through a new gradient-acceptor-doping strategy. By judiciously introducing gradient Cu²⁺-acceptor ions in “soft” (K, Na)NbO₃ (KNN) ceramics, a record combination of properties is achieved — a d₃₃ of 350 pC/N and a Q_m of 340,—the first co-elevation of both parameters exceeding 300 in KNN-based systems. This represents a fourfold improvement in Q_m with only a 10% reduction in d₃₃. Multi-scale structural analysis reveals that the gradient doping retains a “soft” interior for high d₃₃, and creates a “hardened” surface layer that effectively restrains domain switching, accounting for the enhanced Q_m. This work offers a new paradigm for designing high-power ferroelectric ceramics by spatially tailoring functional properties.