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 (Bi 3+ and Ca 2+ ) 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 Ca 2+ and decreasing Bi 3+ , resulting in elevating R phase and reducing T phase. This phenomenon indicates that the contribution of Ca 2+ to increase T R-O is stronger than that from Bi 3+ , while the effect on decreasing T O-T from Ca 2+ is weaker with respect to Bi 3+ during phase boundary formation. The enhancement of T R-O and T O-T is due to the lower electronegativity of Ca 2+ than Bi 3+ which benefits an R phase with high ionicity. There is only a small change in T C and diffusion degree when Bi 3+ is replaced by Ca 2+ , because of the similar substitution of Bi 3+ and Ca 2+ on the A-site. Meanwhile, enhanced O vacancies are due to the lower valence of Ca 2+ than that of Bi 3+ . 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.