B‐Site Substitution in (K, Na)NbO ₃ Lead‐Free Piezoceramics: Distinct Roles of Ti ⁴⁺ and Hf ⁴⁺ in

ABSTRACT

Although the phase boundary engineering is the most effective strategy for enhancing the piezoelectricity of (K, Na)NbO ₃ (KNN)‐based lead‐free piezoelectric ceramics, the regulatory mechanisms of different chemical dopants remain unclear. In particular, the substitution behavior among multiple B‐site elements requires further in‐depth investigation. This study takes 0.95(K _0.48 Na _0.52 )NbO ₃ ‐0.05(Bi _0.5 Na _0.5 )Ti _x Hf _{1‐ x} O ₃ ceramics as a model system to investigate the substitutional effects of Ti ⁴⁺ and Hf ⁴⁺ at the B‐site and its regulatory effects on the multi‐scale structure and electrical properties. The experimental results, supported by first‐principles calculations, suggest that Ti ⁴⁺ promotes lattice contraction and local off‐centering tendency, thus weakening the degree of room‐temperature multiphase coexistence. Furthermore, Ti ⁴⁺ doping leads to grain refinement and homogenization of the domain structure. Although these factors lead to a decrease in piezoelectricity, the temperature stability is significantly enhanced. This work provides insight into how different B‐site dopants regulate the multi‐scale structure of KNN‐based ceramics and provide a new perspective for the rational compositional design of high‐performance lead‐free piezoceramics.