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

Abstract

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.