Giant Dielectric Response In A(Fe_1/2Nb_1/2)O₃(A=Sr, Ca) And SrRCoO₄(R=La, Nd) Ceramics

In the present work, the giant dielectric response and magnetic behavior in A(Fe_1/2Nb_1/2)O₃(A=Sr, Ca) and SrRCoO4(R=La, Nd) ceramics have been investigated together with the physical nature and the structural origin of such giant dielectric response.Dense A(Fe_1/2Nb_1/2)O₃(A=Sr, Ca) ceramics were prepared by a solid state sintering process and the dielectric properties were characterized. Two dielectric relaxations represented by a low temperature dielectric constant step and a high temperature relaxor-like behavior are observed in A(Fe_1/2Nb_1/2)O₃(A=Sr, Ca) ceramics. Both dielectric relaxations show frequency dispersion and follow the thermally activated Arrhenius law. The relaxor-like dielectric relaxation can almost be removed completely by annealing in O₂ for Sr(Fe_1/2Nb_1/2)O₃ ceramics while being suppressed severely for Ca(Fe_1/2Nb_1/2)O₃ ceramics. Therefore, this high temperature dielectric relaxation should be an extrinsic one induced by the ordering of defects. The dielectric constant step is not sensitive to the annealing in O₂, and the intrinsic electronic ferroelectricity induced by the ordering of Fe²⁺ and Fe³⁺ ions has been proposed to be responsible for it.In light of the inference about electronic ferroelectricity in A(Fe_1/2Nb_1/2)O₃, the dielectric properties of SrRCoO₄ (R=La, Nd) were investigated to provide deeper insight. Although the detection of a giant dielectric constant step in SrLaCoO₄ ceramics and a partial dielectric constant flat in SrNdCoO₄ ceramics, detailed analysis can not be made due to the unstable dielectric data caused by the high conductivity in both ceramics. The doping of Al can decrease the conductivity significantly and stable dielectric data were obtained as a result in SrLaCo_0.6Al_0.4O₄ ceramics. The giant dielectric constant step is observed obviously in this modified material and indicates a close activation energy to that of similar dielectric behavior in A(Fe_1/2Nb_1/2)O₃, which stands as a strong evidence for the electronic ferroelectricity in all those materials.The magnetic properties of A(Fe_1/2Nb_1/2)O₃ and SrRCoO₄ were evaluated to understand the electronic ferroelectricity dominated by magnetic Fe and Co ions. A(Fe_1/2Nb_1/2)O₃ system shows the same antiferromagnetic ordering at low temperatures and also slight ferromagnetic ordering below Neel temperature. This result demonstrates the similar electronic configuration of Fe ions, which is consistent with what the similar dielectric constant steps of A(Fe_1/2Nb_1/2)O₃ system caused by the ordering of heterovalent Fe ions suggest. SrRCoO4 shows ferromagnetic transition within the temperature range for giant dielectric constant step as well as magnetoresistance effect with characteristic change at T_C Those results demonstrate the correlation between electric and magnetic mechanism, and the domination of Co ions for both giant dielectric response and magnetic transition, which supports the inference about electronic ferroelectricity.