Frequency dependence of polarization and strain in Bi0.5Na0.5TiO3-SrTiO3/Bi0.5(Na0.8K0.2)0.5TiO3 composites

This study involves the microstructure tailoring of two types of ferroelectric and relaxor composites and their frequency-dependent large-signal properties. The ferroelectric seed of Bi0.5(NaK)0.5TiO3 (BNKT) and a relaxor matrix of 0.74 Bi0.5Na0.5TiO3-0.26SrTiO3 (BNST) with a ratio of 0.1 were synthesized via a solid-state reaction. Two types of BNKT ferroelectric seeds, namely a large-sized template-type (t-BNKT) and small-sized powder-type (c-BNKT), were prepared. Additionally, X-ray diffraction and microstructure analyses revealed that the addition of BNKT to BNST induced the formation of two phases (ferroelectric and relaxor). BNKT/BNST exhibited a combination of relaxor and ferroelectric phases, while the relaxor corresponded to the major phase in pure BNST. The t-BNKT/BNST composite exhibited a large-sized ferroelectric phase in the relaxor matrix with a small population, and the c-BNKT/BNST contained a small-sized ferroelectric phase with a large population of the relaxor matrix in the microstructure. In high-field applications, BNST exhibited pinch-type polarization behavior due to the transition from polar nanocluster to ferroelectric microscale domains in the relaxor matrix. The addition of t-BNKT and c-BNKT accelerated the coexistence of the ferroelectric and relaxor phases, thereby inducing pinch-shaped polarization with high remnant polarization and parabolic strain behavior with high negative strain. The pure BNST and t-BNKT/BNST exhibited a transition from the relaxor to ferroelectric phases at a high-field application over 4 kV/mm. Furthermore, the advent of the ferroelectric phase in c-BNKT/BNST led to a transition from short-range polar ordering to long-range ferroelectric ordering when a lower field (<3 kV/mm) was applied. However, the sample exhibited difficulties in reverse transition upon removal of the applied field. The electromechanical properties of c-BNKT/BNST and t-BNKT/BNST indicated a significant change in the frequency dependence of the polarization and strain, which was related to the presence of the ferroelectric phase. The aforementioned property was evaluated based on the decay in the polarization behavior.