Impact of oxygen vacancy-gradient surface layer on the electrostrain of Sb3+ doped BNKST ceramics

Recently, ultra-large electrostrain exceeding 1% has been observed in various thin ferroelectric ceramics, generating a significant research interest. However, the underlying mechanisms remain controversial. In this work, electrostrain, along with the dielectric and ferroelectric properties of {(K0.2Na0.8)0.5Bi0.5}0.96Sr0.04SbxTi1−xO3−x/2 ceramics, was investigated, and all of them feature a sample's thickness (t) dependence. As t decreased from 0.4 to 0.1 mm, both the permittivity (ε′) and loss tangent (tanδ) diminished, and the maximum polarization (Pmax) decreased, while the electrostrain (S) exhibited a significant increase. X-ray diffraction and x-ray photoelectron spectroscopy analyses revealed structural differences between the bulk and the surface of the ceramics, and surface reconstruction during the poling process. Based on these findings, we propose that the thickness-dependent electric properties are related to hopping oxygen vacancies, which forms an oxygen vacancy-gradient layer on the ceramic surface under the applied electric field. Our work is conducive to understanding the thickness-dependent properties recently discovered in piezoelectric ceramics.