Atomic-scale structural origin of giant strain in Ta-modified BNKT-based relaxor ferroelectrics

Relaxor ferroelectrics with extraordinary properties have aroused continuous research interest due to their extensive applications in functional electrical devices based on their electromechanical coupling. However, the origin of their exceptional electrical performance from an atomic-scale structural perspective is still enigmatic to understand. Here, we implement the aberration-corrected scanning transmission electron microscope (AC-STEM) to capture the atomic-scale evidence of Ta⁵⁺-doped Bi_0.5(Na_0.8K_0.2)_0.5TiO₃-based (BNKT) relaxor ferroelectrics and demonstrate the correlation between the macroscopic properties and microscopic evidence. The annular dark field (ADF) scanning transmission electron microscopy (STEM) images and corresponding δ_Ti-Bi/Na displacement vector maps demonstrate the coexistence of tetragonal (T) and rhombohedral (R) phases in the resulting ceramics. Inhomogeneity and short-range disorder are increased in the Ta-doped sample, resulting in an increase in relaxation. Macroscopic properties, including temperature-dependent dielectric measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and ferroelectric measurements, are in good accordance with the phenomenon observed at the atomic scale. Notably, the BNKTT-4ST-1Ta sample with a pinched hysteresis loop shows a giant strain of 0.57 %, which is almost 3.5 times greater than that of the undoped sample. More interestingly, the cation displacement and oxygen octahedron tilt in the 1Ta sample show regional consistency in multiple directions. This work reveals the relationships between composition, atomic-scale structure, and macroscopic properties, providing effective guidance for the design of relaxor materials for high-displacement actuator applications.