Deciphering the atomic-scale structural origin for large dynamic electromechanical response in lead-free Bi_0.5Na_0.5TiO₃-based relaxor ferroelectrics

Despite the extraordinary electromechanical properties of relaxor ferroelectrics, correlating their properties to underlying atomic-scale structures remains a decisive challenge for these "mess" systems. Here, taking the lead-free relaxor ferroelectric Bi_0.5Na_0.5TiO₃-based system as an example, we decipher the atomic-scale structure and its relationship to the polar structure evolution and large dynamic electromechanical response, using the direct atomic-scale point-by-point correlation analysis. With judicious chemical modification, we demonstrate the increased defect concentration is the main driving force for deviating polarizations with high-angle walls, leading to the increased random field. Meanwhile, the main driving force for deviating polarizations with low-angle walls changes from the anti-phase oxygen octahedral tilting to the multidirectional A-O displacement, leading to the decreased anisotropy field. Benefiting from the competitive and synergetic equilibrium of anisotropic field versus random field, the facilitated polarization rotation and extension versus facilitated domain switching are identified to be responsible for the giant electromechanical response. These observations lay a foundation for understanding the "composition-structure-property" relationships in relaxor ferroelectric systems, guiding the design of functional materials for electromechanical applications.