Revealing the complex nature of bonding in binary high-pressure compound FeO$_2$.

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides resulting in numerous compounds forming homologous series $n$FeO$\cdot m$Fe$_2$O$_3$ and the appearance of FeO$_2$. Here, based on the results of \emph{in situ} single-crystal X-ray diffraction, M\"ossbauer spectroscopy, X-ray absorption spectroscopy, and DFT+dynamical mean-field theory calculations we demonstrate that iron in high pressure cubic FeO$_2$ and isostructural FeO$_2$H$_{0.5}$ is ferric (Fe$^{3+}$), and oxygen has a formal valence less than two. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.