Metal-insulator transition and the role of electron correlation in FeO2

Iron oxide is a key compound to understand the state of the deep Earth. It has been believed that previously known oxides such as FeO and Fe2O3 will be dominant at the mantle conditions. However, the recent observation of FeO2 shed another light to the composition of the deep lower mantle (DLM), and thus understanding of the physical properties of FeO2 will be critical to model the DLM. Here, we report the electronic structure and structural properties of FeO2 by using density functional theory and dynamic mean-field theory. The crystal structure of FeO2 is composed of Fe2+ and O22- dimers, where the Fe ions are surrounded by the octahedral O atoms. We found that FeO2 shows a metal-insulator transition (MIT) under high pressure. The MIT is not a Mott type but a band insulator type which is driven by the O2 dimer bond length change. However, the correlation effect of Fe 3d orbitals should be considered to correctly describe O2 dimer bond length of FeO2 and the MIT.