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 μC/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.

Figure 1

(a) The room-temperature XRD pattern of Fe-doped BTO ceramics; (b) Rietveld refinement plots for Fe-doped BTO ceramics; (c) Raman spectroscopy of Fe-doped BTO ceramics; and (d) the deconvolution fitting for Fe-doped BTO ceramics.

Figure 2

(a)–(c) The SEM images for Fe-doped BTO ceramics; (d)–(f) the mean grain size map for Fe-doped BTO ceramics.

Figure 3

(a)–(c) Temperature dependence of the dielectric permittivity and dielectric loss in the Fe-${\mathrm{BaTiO}}_3$ ceramics; (b)–(d): the dielectric constant susceptibility of Fe-doped BTO as a function of high temperature for various frequencies.

Figure 4

$Z^{*}$ plot of (a) BTO, (b) $x\text{Fe-BTO}$, and (c) $4x$/3Fe-BTO ceramics as a function of temperature. Complex-plane impedance plots for (d) BTO (e) $x\text{Fe-BTO}$, and (f) $4x$/3Fe-BTO ceramics at $400{}^{\circ }\mathrm{C}$ with z-view fitting results. Illustration in (e) shows an equivalent circuit for the ceramic samples.

Figure 5

TSDC spectra for Fe-doped BTO ceramics with different polarization field: (a) BTO, (b) $x\text{Fe-BTO}$, and (c) $4x$/3 Fe-BTO.

Figure 6

The O $1s$ XPS spectra of (a) BTO, (b) $x\text{Fe-BTO}$, and (c) $4x$/3Fe-BTO, and the Ti $2p$ XPS spectra of (d) BTO, (e) $x\text{Fe-BTO}$, and (f) $4x$/3Fe-BTO.

Figure 7

The EPR spectra for Fe-doped BTO.

Figure 8

The hysteresis loop of Fe-doped BTO ceramics at room temperature.

Figure 9

(a): The room-temperature flexoelectric coefficient for Fe-doped BTO ceramics; (b) the variable temperature flexoelectric coefficient for Fe-doped BTO ceramics; (c) the variable temperature flexoelectric coupling coefficient for Fe-doped BTO ceramics.