Abstract: Recently, large apparent strain resulting from bending deformation in thin piezoelectric ceramics has attracted significant attention. However, whether oxygen vacancies or defect dipoles contributes to such macroscopic deformation remains controversial. In this study, bending deformation in (Na_0.81K_0.19)_0.5Bi_0.5Nb_0.01Ti_0.99O₃ ceramics is investigated, and the underlying microscopic mechanisms are clarified. X-ray photoelectron spectroscopy analysis indicates a gradient distribution of oxygen vacancy concentration across both the thickness and radial directions of the ceramic. And piezoresponse force microscopy confirms that the top and bottom surfaces of the sample exhibit different domain mobility. Furthermore, it is demonstrated that the oxygen vacancy concentration gradient along the thickness direction plays a decisive role in inducing the bending deformation. Based on these findings, a mechanism for bending deformation is proposed. The gradient in oxygen vacancy concentration along the thickness direction of the ceramic induces spatially non-uniform domain switching during polarization, leading to differential contraction between the top and bottom surfaces and ultimately resulting in macroscopic bending deformation. This study establishes a comprehensive mechanistic chain spanning from oxygen vacancy migration (microscopic) to inhomogeneous domain switching (mesoscopic) and finally to macroscopic bending deformation, providing a crucial theoretical foundation for the design of piezoelectric materials with large electrostrain.

Key words: Lead-free piezoelectric ceramics, Electrostrain, Bending deformation, Oxygen vacancy