Origin of defects induced large flexoelectricity in ferroelectric ceramics

Defects have been regarded as playing a critical role in the functionalities of many solid dielectrics. However, the contribution of defects to the specific coupling between strain gradient and electric polarization (i.e., flexoelectricity) has not yet been thoroughly understood. Herein, we selected the typical ferroelectric ${\mathrm{BaTiO}}_{3}$ (BTO) ceramics and introduced oxygen vacancies and trapped charge defects by using stoichiometric and nonstoichiometric Fe dopants, respectively. Compared with the pure BTO ceramics, the flexoelectric coefficients of stoichiometric Fe-doped BTO ceramics were increased by fivefold (from 9.5 to 65 \ensuremath{\mu}C/m) while that of the nonstoichiometric counterparts almost keep stable. The results show that the oxygen vacancies rather than trapped defects make a remarkable contribution to the enhancement of flexoelectricity, and this is explained by the reorientation of the defect dipoles formed by the oxygen vacancies. The result presented in this work not only benefits the understanding of the mechanism of flexoelectricity but also provides a feasible strategy to design flexoelectric materials and related devices with high flexoelectric coefficients.