Adaptive ferroelectric states in KNN-based piezoceramics: Unveiling the mechanism of enhancing piezoelectric properties through multiple phase boundary engineering

Developing high-performance lead-free piezoceramics, such as (K,Na)NbO₃ (KNN) material, is critical to achieving environmental sustainability in next-generation electromechanical devices. Although substantial advancements have been made in KNN-based ceramics, a general coherent framework is lacking that links microscopic structure to the enhancement of macroscopic properties. Our findings indicate that the enhanced performance of KNN-based ceramics in multiphase coexistence boundaries can be attributed to the formation of self-adjusting nanodomains in response to strong local structural heterogeneity. The self-adjusting behavior of domain structure is an outcome of minimizing the local stress generated by the lattice mismatch within KNN-based ceramics, which conforms to the thermodynamic principles that favor minimizing total free energy. Guided by this strategy, the present work achieves a significant improvement of electromechanical properties, including a piezoelectric coefficient (d₃₃) of ∼585 pC N⁻¹, an electromechanical coupling factor (k_p) of ∼62 %, and a figure of merit (d₃₃ × g₃₃) of ∼12.78 ×10⁻¹² m² N⁻¹. This study offers a new strategy for developing lead-free piezoceramics through dedicated design of novel phase boundaries.