  The discovery of FeO$_{2}$ containing more oxygen than hematite
(Fe$_{2}$O$_{3}$) that was previously believed to be the most oxygen rich iron
compounds, has important implications on the study of the deep lower mantle
compositions. Compared to other iron compounds, there are limited reports on
FeO$_{2}$ making studies of its physical properties of great interest in
fundamental condensed matter physics and geoscience. Even the oxidation state
of Fe in FeO$_{2}$ 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 FeO$_{2}$ consists of Fe(II) and peroxide
O$_{2}^{2-}$. Our study suggests that localized nature of both Fe 3$d$ orbitals
and O$_{2}$ molecular orbitals should be correctly treated for unveiling the
structural and electronic properties of FeO$_{2}$.
