Comparison of contribution to phase boundary from A-site aliovalent dopants in high-performance KNN-based ceramics

Environmentally friendly potassium sodium niobate (KNN)-based ceramics are potential electronic functional materials due to multiphase coexistence. Aliovalent doping on the A-site with different ions plays a key role in phase boundary engineering. However, the difference of contribution to the phase boundary from various A-site dopants is not clear in multielement high performance KNN-based ceramics. Herein, the individual contribution to phase structure and comparison of typical aliovalent ions (Bi³⁺ and Ca²⁺) on the A-site, are considered in terms of influence on electrical properties. Within a maintained rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary at room temperature, both phase transition temperatures for rhombohedral-orthorhombic (T_R-O) and orthorhombic-tetragonal (T_O-T) gradually enhance with increasing Ca²⁺ and decreasing Bi³⁺, resulting in elevating R phase and reducing T phase. This phenomenon indicates that the contribution of Ca²⁺ to increase T_R-O is stronger than that from Bi³⁺, while the effect on decreasing T_O-T from Ca²⁺ is weaker with respect to Bi³⁺ during phase boundary formation. The enhancement of T_R-O and T_O-T is due to the lower electronegativity of Ca²⁺ than Bi³⁺ which benefits an R phase with high ionicity. There is only a small change in T_C and diffusion degree when Bi³⁺ is replaced by Ca²⁺, because of the similar substitution of Bi³⁺ and Ca²⁺ on the A-site. Meanwhile, enhanced O vacancies are due to the lower valence of Ca²⁺ than that of Bi³⁺. Then, electrical properties including ferroelectricity, piezoelectricity and strain, retain high values originating from the maintained R-O-T phase boundary. Moreover, improved stability of piezoelectricity and strain under changing temperature, are achieved based on enhanced T_O-T. Thus, this work provides an effective method to further optimize multiphase structures via appropriate doping in KNN-based ceramics.