Preferential B-Site Fe Substitution Enables Defect-Mediated Grain Growth and Domain Engineering in (K, Na)NbO ₃ -Based Lead-Free Ceramics

(K, Na)NbO 3 (KNN) ceramics have been extensively studied as a leading lead-free piezoelectric candidate, with their performance typically tailored through chemical modification. Despite the widespread use of Fe 2 O 3 as a sintering aid, the site occupancy of the Fe ions within the KNN lattice remains unclear. The specific physical mechanisms underlying their subsequent impact on material properties are still subjects of debate. In this work, we employed three different doping strategies (i.e., A-site doping, B-site doping, and introduction as a sintering aid) to investigate the occupancy tendency mechanism of the Fe ions. Combined experimental results and first-principles calculations reveal a strong preference for Fe ions to occupy the B site in KNN-based ceramics. The synergistic effect of oxygen vacancy-promoted grain growth and enlarged ferroelectric domains is found to be responsible for the enhanced piezoelectric performance. These findings provide critical insights into the site occupancy mechanisms of Fe ions, offering a detailed framework for further performance optimization in KNN-based ceramics.