Simultaneous Enhancement of Piezoelectricity and Mechanical Quality Factor in Hard‐Type KNN‐Based Lead‐Free Ceramics via Hierarchical Domain Architecture

Abstract High mechanical quality factor (Q_m) and low mechanical loss are crucial for the performance of piezoelectric materials in high‐power applications. Although potassium sodium niobate (KNN) based ceramics are promising candidates for lead‐free piezoelectric materials due to their favorable piezoelectric performance and environmental benefits, they traditionally face a trade‐off between high piezoelectric coefficient (d₃₃) and Q_m. To address this long‐standing issue, a multivalent acceptor codoping strategy is presented that innovatively induces hierarchical domain structure in hard‐type KNN‐based piezoceramics, significantly increasing domain wall density and pinning strength while maintaining well‐ordered ferroelectric domain structure, thereby achieving a simultaneous enhancement of Q_m, d₃₃ and thermal stability. The optimized composition possesses a high Q_m up to 948 with a d₃₃ of 132 pC/N, representing improvements of 80% and 25% compared to Cu‐doped samples, and maintaining outstanding performance with Q_m > 800, d₃₃ > 130 pC/N across a broad temperature range of 25–175 °C. Furthermore, our sample retained high Q_m performance under high‐power conditions, outperforming several commercial lead‐based ceramics. These results provide new insights into the defect‐domain interaction of hard‐type KNN‐based ceramics and demonstrate their potential for lead‐free high‐power piezoelectric applications.