Large Strain Response in 0.99(Bi0.5Na0.4K0.1)TiO3-0.01(KxNa1-x)NbO3 Lead-Free Ceramics Induced by the Change of K/Na Ratio in (KxNa1-x)NbO3

Influence of K/Na ratio in (KxNa1-x)NbO3 on the ferroelectric stability and consequent changes in the electrical properties of 0.99(Bi0.5Na0.4K0.1)TiO3-0.01(KxNa1-x)NbO3 (BNKT-KxNN) ceramics were investigated. Results showed that change of K/Na ratio in KNN induces a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long-range ferroelectric order, and correspondingly adjusts the ferroelectric-relaxor transition point TF-R to room temperature. Accordingly, giant strain of 0.46% (corresponding to a large signal d(33)* of 575pm/V) which is comparable to that of Pb-based antiferroelectrics is obtained at a K/Na ratio of 1, and the emergence of large strain response induced by the change of K/Na ratio of KNN can be well explained by the correlation between the position of ferroelectric-ergodic relaxor phase boundary in the BNKT-KxNN system and the tolerance factor t of the end number (KxNN). In situ high-energy X-ray scattering experiments with external field reveals that the large strain response in the studied system is likely related to the electric field-induced distortion from the pseudocubic structure.