Structural phase transition, phase purity, inter-/intra-grain electrical parameters and local energy barrier profile as a function of aliovalent and isovalent substitution in BiFeO3

Abstract

The presence of space charges deteriorates the electrical properties of multiferroics and limits their use in commercial devices. However, identification of the conduction mechanisms and their origin is a very challenging task. In this study electrical and dielectric responses of pure and substituted BiFeO3 samples are studied in detail by intentionally controlling the formation of oxygen vacancies and space charges via a variety of isovalent and aliovalent substitutions. Polydispersive inter- and intra-grain hopping mechanisms are traced as a function of substitution to highlight finer details associated with their origin. Additionally, for quantification purposes, equivalent circuit analysis of Nyquist and Cole-Cole plots is carried out to extract intra/inter grain parameters, such as resistivity and permittivity, and to establish the polydispersive nature of the conduction mechanisms. With the consideration of all observed conductivity processes and their activation energies, an overall schematic energy barrier diagram is plotted. Equivalent circuit analysis is also utilized for the estimation of grain and grain boundary capacitance and resistance values. Results obtained after dielectric analysis, Cole-Cole and Nyquist plots are confirmed based on sample compositions and discussed in terms of the ionic radii mismatch and valence state of the substituents and indicate that both parameters play an effective role in controlling the mechanisms associated with the oxygen content in the sample.