  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, 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 DLM.
Here, we report the electronic structure and structural properties of FeO2 by
using density functional theory (DFT) and dynamic mean field theory (DMFT). The
crystal structure of FeO2 is composed of Fe2+ and O2 2- dimers, where the Fe
ions are surround by the octahedral O atoms. We found that the bond length of
O2 dimer, which is very sensitive to the change of the Coulomb interaction U of
Fe 3d orbital, plays an important role in determining the electronic
structures. The band structures of DFT+DMFT show that the metal-insulator
transition is driven by the change of U and pressure. We suggest that the
correlation effect should be considered to correctly describe the physical
properties of FeO2 compound.
