Dielectric abnormality and ferroelectric asymmetry in W/Cr co-doped Bi4Ti3O12 ceramics based on the effect of defect dipoles

In this paper, a sort of W/Cr co-doped Bi4Ti3O12 ceramics with different additional amount of Cr2O3: Bi4Ti2.95W0.05O12.05 + y wt. % Cr2O3 (BITW-yCr, y = 0–0.4) were prepared by a conventional ceramic process. XRD analysis for BITW-yCr ceramics firstly reveal that the formation of orthorhombic Bi4Ti3O12 was accompanied by the output of less cubic Bi2Ti2O7 as the second phase. On the dielectric response curves of samples, there are two abnormal variations that a dielectric “hunch” with the relaxation characteristic occurs around 500 °C at low frequencies, and the dielectric peak at Curie temperature presents an obvious dispersity as its intensity is significantly decreasing with the increase of frequency. Here, a physical mode for describing the formation process and acting mechanism of defect dipoles was established. In the pseudo-perovskite layer of BITW-yCr ceramics, the electronegative cation defects tend to associate with the electropositive oxygen vacancies, forming the electric defect dipoles. The orientated polarization of defect dipoles driving by the external electric filed contributed to the dielectric relaxation process based on the activated motion of oxygen vacancy at high temperatures, besides the inability of ionic polarization, the interference effect of defect dipoles on the short-range interaction of intrinsic dipoles may be another reason for the dielectric dispersion. On the other hand, the internal bias field built by the reorientated defect dipoles is considered to cause the asymmetry of hysteresis loops for ferroelectrics, and the addition of Cr2O3 (y) is found to increase the internal bias field of BITW-yCr ceramics. Additionally, it is also observed to decrease the temperature coefficient of dielectric constant of samples. Finally, profited from a lower coercive field and a larger spontaneous polarization, the resulting large remanent polarization (Pr = 17 μC/cm2) contributes to a high piezoelectricity (d33 = 28 pC/N) for the ample at y = 0.2. •Cation defects and oxygen vacancies are associated to form defect dipoles.•The orientated polarization of defect dipoles responds a dielectric “hunch”.•The interference effect of defect dipoles may cause the dielectric dispersity.•The internal bias field built by defect dipoles induces the ferroelectric asymmetry.•The addition of Cr2O3 decreases the temperature coefficient of dielectric constant.