Tuning functional properties via Fe/Al ratio in the franklinite–gahnite (ZnFe2O4-ZnAl2O4) solid solution series

In this study, a comprehensive investigation of the structure-property relationship in the solid solution series ZnFe 2-x Al x O 4 with x = 0.0 to 2.0 is provided. Measurements of the electrical conductivity, optical band gap, magnetic susceptibility, and other relevant parameters are discussed in dependence on the aluminum concentration. As expected, the band gap increases from roughly 2.0 eV to 4.2 eV while electronic conductivity and magnetic susceptibility decrease the more Fe 3+ is substituted by Al 3+ . In addition, it was found that pure zinc ferrite, at least, exhibits conductivity dependent on the partial pressure of oxygen at temperatures above 600 °C. This could be an indication of possible oxygen-ion conductivity in the high-temperature range. Furthermore, special focus is put on the potential application of ZnFe 2-x Al x O 4 with x ≤ 1.0 as active material at the positive electrode of zinc-metal batteries. Zinc (de)insertion is investigated by cyclovoltammetric measurements in spinel|buffered aq. electrolyte|Zn cells. Results show that apart from zinc ions a considerable amount of protons can be reversibly co-inserted into the material, making it a possible contender as a stable active material. • Increasing Al concentration reduces total conductivity, magnetic susceptibility. • Optical band gap increases with Al concentration. • Increased ionic conductivity at low Al concentrations at room temperature. • Above 600 °C indications of dependence of electronic conductivity on pO 2. • Reversible proton and zinc ion (de)insertion with aqueous electrolyte.