Mr. Roni Eppstein
Transition metal oxide spinels are widely used in a variety of applications such as photovoltaics, solid oxide fuel cells, electronic devices, and more. Conduction in these materials is often described by small-polarons hopping between octahedrally coordinated (Oh) cations and is thus dependent on the cation distribution and stoichiometry. Ternary spinel oxides, where the cations are of different elements, challenge this model by introducing nonequivalent Oh sites, leading to multiple possible pathways for charge transport. In this work, we computationally investigated the electronic properties in epitaxially strained MnxFe3-xO4 (MFO), a ternary spinel that undergoes a significant shift from the half-metallic magnetite (x = 0, Fe3O4) to semiconducting hausmannite (x = 3, Mn3O4), in concert with transitions in both its cation distribution and crystal structure, going from cubic to tetragonal. We present density functional theory results in comparison with experimental observations such as the preference for inversion of the spinel structure in MFO, as well as projected density of states analysis, highlighting the possible hopping pairs in the system. Small-polaron formation and transport, modeled according to Marcus theory, suggest a preference for transport along Mn pathways over Fe pathways, consistent with experimental observations of site occupation and conductivity. Finally, we present results on the effects of oxygen deficiency in MFO on its electronic and transport properties.