Defect chemistry and electrical properties of BiFeO3

Abstract

BiFeO₃ attracts considerable attention for its rich functional properties, including room temperature coexistence of magnetic order and ferroelectricity and more recently, the discovery of conduction pathways along ferroelectric domain walls. Here, insights into the defect chemistry and electrical properties of BiFeO₃ are obtained by in situ measurements of electrical conductivity, σ, and Seebeck coefficient, α, of undoped, cation-stoichiometric BiFeO₃ and acceptor-doped Bi_1−xCa_xFeO_3−δ ceramics as a function of temperature and oxygen partial pressure pO₂. Bi_1−xCa_xFeO_3−δ exhibits p-type conduction; the dependencies of σ and α on pO₂ show that Ca dopants are compensated mainly by oxygen vacancies. By contrast, undoped BiFeO₃ shows a simultaneous increase of σ and α with increasing pO₂, indicating intrinsic behavior with electrons and holes as the main defect species in almost equal concentrations. The pO₂-dependency of σ and α cannot be described by a single point defect model but instead, is quantitatively described by a combination of intrinsic and acceptor-doped characteristics attributable to parallel conduction pathways through undoped grains and defect-containing domain walls; both contribute to the total charge transport in BiFeO₃. Based on this model, we discuss the charge transport mechanism and carrier mobilities of BiFeO₃ and show that several previous experimental findings can readily be explained within the proposed model.