Engineering the nature of polarization dynamics in lead-free relaxors based on (Bi_1/2Na_1/2)TiO₃

(Bi_1/2Na_1/2)TiO₃ (BNT) is a lead-free ferroelectric material, which has shown promising electromechanical properties and energy storage capacities. These attractive functionalities and property performances are attributed to the relaxor behavior of BNT. However, the nature of the dielectric relaxation is not well understood in these materials, and the physical meaning of some important parameters associated with the permittivity is still under debate. In this Letter, we focus on the dielectric “shoulder,” T_s, that is readily seen in the ε′-T curve of every BNT-based relaxor. It is found that the T_s is controlled by not only the typical compositional engineering but also by the thermal, electrical, or mechanical history. From a moderate temperature (≤250 °C), a sample can be rapidly cooled to room temperature or slowly cooled under an electrical bias or a mechanical bias in the form of a compressive stress. All three treatments lead to a nearly identical effect, which is to alter the T_s with respect to rest of the ε′-T curve that remains unperturbed. Therefore, the internal stress is identified to be a general perturbance to the polarization dynamics. Finally, the “breathing” model is revisited to interpret the physical meaning of T_s for these BNT materials under these metastable conditions.