Gradient‐Acceptor‐Doping Breaks the Piezoelectric Trade‐Off in Lead‐Free (K, Na)NbO ₃ Ceramics

Abstract Achieving a high piezoelectric coefficient ( d 33 ) 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 Cu 2+ ‐acceptor ions in “soft” (K, Na)NbO 3 (KNN) ceramics, a record combination of properties is achieved — a d 33 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 d 33 . Multi‐scale structural analysis reveals that the gradient doping retains a “soft” interior for high d 33 , 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.