Enhanced piezoelectric performance and thermal stability in KNN-BNZ ceramics modified by CaHfO3

Abstract (K, Na)NbO 3 (KNN) is regarded as a promising lead-free piezoelectric candidates due to its high Curie temperature and environmental compatibility. However, its piezoelectric performance remains inferior to Pb-based ceramics, mainly due to the rigid lattice and high coercive field that restrict domain switching. In this study, CaHfO 3 was introduced to modify the perovskite structure of KNN through a homologous co-doping mechanism. The simultaneous substitution of Ca 2+ and Hf 4+ modifies the local bonding environment and induces lattice softening, which effectively reduces the coercive field and facilitates polarization switching. At the optimal composition of 0.95(K 0.5 Na 0.5 )NbO 3 -0.02(Bi 0.5 Na 0.5 )ZrO 3 -0.03CaHfO 3 (0.95KNN-0.02BNZ-0.03CHO), the ceramics exhibit significant improvements in both ferroelectric and piezoelectric properties, while maintaining a balanced Curie temperature. This work provides clear evidence that structural softening induced by Ca-Hf incorporation provides an effective materials design strategy for enhancing the performance of lead-free KNN-based ceramics, while the compositional limits and long-term device-level behavior require further investigation.