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Giant electrocaloric effect enhancement due to the polarization flip and impact of Mn4+ doping on the dielectric, ferroelectric properties in 0.7BiFeO3-0.3BaTiO3 ceramics

As a kind of ferroelectric and antiferromagnetic coexistent multi-ferroic material, BiFeO3 (BFO) has a theoretical saturation polarization over 100 mC/cm2, and a Curie temperature of 830 °C, which may offer a huge electrocaloric effect. However, due to the evaporation of Bi2O3 during the sintering process at high temperatures, and the variation of chemical valence of iron ions, there are lots of point defects and also a large leakage current existed in BFO, which make the ferroelectricity of BFO hard to be presented and measured. Although the forming of solid solution with BaTiO3 (BTO) or other oxide ferroelectrics may mitigate the leakage current, high loss tangent is still existed. In this work, it is trying to address this issue in terms of adding manganese ions in the BFO-BTO solid solution. 0.7(BFO)-0.3(BTO)+xwt%MnO2 ceramics were prepared using the conventional solid-state reaction at high temperatures. The microstructure, dielectric and ferroelectric characteristics were investigated via doping with different Mn4+ions. Results indicated that the crystallographic structure was rhombohedral and pseudocubic phase coexistence. It was observed that certain Mn4+ ionic content may lead to the reduction of loss tangent and leakage current for BFO-BTO ceramics, which was accounted for the compensation of Mn4+ ions with the oxygen vacancies. In addition, it was procured that the 0.7BFO-0.3BTO+0.5wt%MnO2 ceramic offered a maximum polarization of 50.53 μC/cm2 at 100 kV/cm. Finally，the direct approach was used to measure the electrocaloric effect. It was found that using Lu et al. proposed polarization flip method, the ECE temperature change was observed to increase almost 8 times when changing the electric field from 0 to -30 kV/m with respect to the changing of electric field from 30 kV/cm to 0. This verified that the Lu et al’s method was also applicable to polycrystalline first-order phase transition ferroelectrics.