Achieving synergistic electromechanical and electrocaloric responses by local structural evolution in lead-free BNT-based relaxor ferroelectrics

Self-actuating cooling materials based on synergistic electromechanical (EM) and electrocaloric (EC) responses are expected to lead next-generation of intelligent cooling devices. However, most current materials with the two responses present poor performance, narrow operation-temperature span and high driving electric field, and even contain toxic lead. Here, environmentally benign lead-free Bi0.5Na0.5TiO3-based (BNT) system was designed to achieve large combined response of two effects in a large temperature span by adjusting intrinsic A-site Na+/K+ ratio to induce relaxor evolution. Simultaneously, high EC strength and large electrostrictive coefficient over a wide temperature range were obtained to resolve these challenges. The local-structural evolution is studied in detail to provide a theoretical basis for future application of this intelligent material. A novel asymmetric EC response presenting a larger endothermic peak was realized at a low electric field (<3 kV mm−1), which can provide a guideline for the development of low-driving-field EC materials to a certain extent. The phase-field simulation was performed to predict this unique behavior by describing the corresponding dipole coupling states under various electric fields.