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 (TR–O) and orthorhombic–tetragonal (TO–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 TR–O is stronger than that from Bi3+, while the effect on decreasing TO–T from Ca2+ is weaker with respect to Bi3+ during phase boundary formation. The enhancement of TR–O and TO–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 TC 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 TO–T. Thus, this work provides an effective method to further optimize multiphase structures via appropriate doping in KNN-based ceramics.