Design of Domain Structure in Hot-Pressed (K Na1-)NbO3 Ceramics for Electromechanical Performance

Stoichiometry profoundly influences the microstructure and functional properties of ferroelectric ceramics, yet intrinsic microscopic effects of the K/Na ratio in pristine (K,Na)NbO 3 (KNN) remain insufficiently understood due to extrinsic interference from density and doping variations in conventional sintered KNN ceramics. Here, we systematically elucidate the intrinsic role of K/Na stoichiometry on domain configurations, related electromechanical and high-power properties in highly densified pristine KNN ceramics by hot pressing. Our findings reveal a stark stoichiometric contrast. K-rich compositions develop long-range-ordered checkerboard-like domains, while Na-rich ones yield fragmented, watermark-like domains. This stoichiometry-induced domain evolution is attributed primarily to changes in NbO 6 octahedral distortion and vacancy concentration with various K/Na ratios, critically affecting polarization continuity and domain wall mobility. Electrical and high-power measurements further show that Na-rich compositions exhibit strong domain wall pinning, which suppresses nonlinear losses and enables higher sustainable vibration velocities under high-power excitation, whereas K-rich ceramics suffer rapid performance degradation due to excessive domain mobility. An equal K/Na ratio achieves an optimal balance between domain mobility and pinning, resulting in enhanced piezoelectric response together with improved high-power stability. This study underscores the importance of precisely controlling A-site stoichiometry to further engineer structures and functionalities in lead-free piezoelectric ceramics.