Impact of inversion and non-stoichiometry on the transport properties of mixed zinc-cobalt ferrites

Abstract

Metal spinel ferrites enable magnetic, electrical and (photo-)catalytic device applications. For example, tailoring the material's composition, the degree of inversion as well as the non-stoichiometry of the spinel enables controlling its electrical conductivity. The latter two, however, are rarely considered despite their vast impact on the structure–property relationship. Here, we elucidate their importance by carefully examining the temperature dependence (T = 600 °C to 50 °C) of the electrical conductivity of quaternary Zn_(1−x)Co_xFe₂O₄ ferrites under ambient and reducing atmospheric conditions. We show that the substitution of Co for Zn in bulk ZnFe₂O₄ results in a significant enhancement of the activation energy E_A from 0.36 to 0.55 eV under an ambient atmosphere as mixed hopping between Co²⁺/Fe³⁺ sites dominates in Co containing ferrites, while the electrical conductivity in ternary ZnFe₂O₄ arises from electrons hopping between Fe²⁺/Fe³⁺ octahedral sites. More importantly, we demonstrate that hopping mainly occurs between Fe²⁺/Fe³⁺ octahedral sites (E_A < 0.1 eV) under reducing conditions independent of the Co content as the release of oxygen increases the concentration of electrons. Our results highlight that controlling the non-stoichiometry is important for tuning of the electrical properties and essential for taking full advantage of quaternary ferrites in device applications.