Electronic spin transition in FeO2: Evidence for Fe(II) with peroxide O22-

The discovery of FeO2 containing more oxygen than hematite (Fe2O3), which was previously believed to be the most oxygen rich iron compound, has important implications for the study of deep lower mantle compositions. Compared to other iron compounds, there are limited reports on FeO2, making studies of its physical properties of great interest in fundamental condensed matter physics and geoscience. Even the oxidation state of Fe in FeO2 is the subject of debate in theoretical works and there have not been reports from experimental electronic and magnetic properties measurements. Here, we report the pressure-induced spin state transition from synchrotron experiments and our computational results explain the underlying mechanism. Using density functional theory and dynamical mean field theory, we calculated spin states of Fe with volume and Hubbard interaction U change, which clearly demonstrate that Fe in FeO2 consists of Fe(II) and peroxide O. Our paper suggests that the localized nature of both Fe 3d orbitals and O-2 molecular orbitals should be correctly treated for unveiling the structural and electronic properties of FeO2.