Reversible phase transitions and enhanced electrostrain in BNST-xFN ceramics under electric and thermal stimuli

Ferroelectric materials based on (Bi0.5 Na0.5)TiO3 are well-known for their outstanding chemical stability and exceptional electrical properties, particularly their large electrostrain response under applied electric fields, positioning them as promising candidates for precision actuator applications. In this study, we investigate the electrical and structural responses of lead-free (Bi_0.38 Na_0.38 Sr_0.24)Ti_1-x (Fe_0.5 Nb_0.5)_xO₃ (BNST-xFN) ferroelectric ceramics under the combined effects of temperature and electric field. Using in-situ electric field and variable temperature Raman spectroscopy, piezoelectric force microscopy, and comprehensive dielectric and ferroelectric property evaluations, we explore the evolution of structural transformations, polarization behavior, and macroscopic property changes in ceramics with different initial phase structures under thermal and electrical stimuli. Notably, the BNST-0.01FN composition, located near the boundary between the non-ergodic relaxor and ergodic relaxor phases, exhibits a remarkable room-temperature electrostrain of 0.37%, driven by a reversible electric field-induced nonpolar-to-polar phase transition. Upon heating, as the BNST ceramic approaches the phase boundary, a prominent electrostrain (~0.38%) is observed near the temperature of the ferroelectric-to-relaxor phase transition (T_FR, ~60 °C) under the electric field. This study combines in-situ microstructural analysis with macroscopic ferroelectric characterization, providing a deeper understanding of the dynamic coupling between microscopic fields and macroscopic electrical properties, and offering valuable insights for the design of high performance lead-free ferroelectric ceramics.