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 degrees C to 50 degrees 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 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.

Automated summary

Metal spinel ferrites play important roles in magnetic, electrical and (photo-)catalytic device applications. The electrical conductivity of these ferrites can be controlled by tailoring their composition, degree of inversion, and non-stoichiometry. The substitution of Co for Zn in ZnFe2O4 ferrites significantly affects the activation energy and the type of hopping mechanism, depending on the atmospheric conditions. Our findings emphasize the importance of controlling non-stoichiometry for tuning the electrical properties of quaternary ferrites.