On the Interplay Between Oxygen Vacancies and Small Polarons in Manganese Iron Spinel Oxides

Ternary spinel oxides are promising materials due to their potentially versatile properties resulting from the disorder inherent in their crystal structure. To fully unlock the potential of these materials, a deeper understanding of their electronic structures, both as pristine and defective crystals, is required. In the present work, we investigate the effects of oxygen vacancies on the electronic structure and charge transport properties of the ternary spinel oxide Mn Fe_3- O₄, modeled on epitaxial thin films of the material, using density functional theory + U (DFT + U). The formation energy of a single oxygen vacancy in the spinel cell is found to be large and unaffected by changes in stoichiometry, in agreement with experimental results. We find that the immediate vicinity of the vacancy has a marked impact on the formation energy. In particular, Mn cations are found to be preferred over Fe as sites for charge localization around the vacancy. Finally, we examine the charge transport in the defective cell using the formalism of Marcus theory and find that the activation barrier for electron small-polaron hopping between sites not adjacent to the vacancy is significantly increased, with a large driving force toward sites that reside on the same (001) plane as the vacancy. Hence, vacancies delay charge transport by increasing the activation barrier, attributed to a rearrangement of vacancy-released charge on the cations immediately neighboring the vacancy site. These results highlight the impact of oxygen vacancies on charge transport in spinel oxides.