Symmetry of the Underlying Lattice in (K,Na)NbO₃-Based Relaxor Ferroelectrics with Large Electromechanical Response

The piezoelectric constant (d₃₃) and converse piezoelectric coefficient (d₃₃*) of a piezoelectric material are critically important parameters for sensors and actuators. Here, we simultaneously achieve a high d₃₃ of 595 ± 10 pC/N and a large d₃₃* of ∼776 ± 20 pm/V in (K,Na)NbO₃ (KNN)-based ceramics, which exceed those of PZT5H and PZT4 ceramics, presenting good potential for practical piezoelectric applications. Moreover, the ceramic exhibits a relaxor-like and diffuse dielectric behavior due to the occurrence of local heterogeneity. According to the experiments and atomic resolution polarization mapping by Z-contrast imaging, hierarchical architecture of nanodomains and even smaller polar nanoregions with multiphase coexistence caused by compositional modification is the structural origin of the enhanced piezoelectric properties in this work. This work would pave a practical way to future applications of lead-free KNN-based ceramics.