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

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)CoxFe2O4 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 EA 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 (EA < 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. This journal is