Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides resulting in numerous compounds forming homologous series nFeO⋅mFe2O3 and the appearance of FeO2. 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 FeO2 and isostructural FeO2H0.5 is ferric (Fe3+), 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.
@article{arxiv.1905.05497,
title = {Revealing the complex nature of bonding in binary high-pressure compound FeO$_2$},
author = {E. Koemets and I. Leonov and M. Bykov and E. Bykova and S. Chariton and G. Aprilis and T. Fedotenko and S. Clément and J. Rouquette and J. Haines and V. Cerantola and K. Glazyrin and C. McCammon and V. B. Prakapenka and M. Hanfland and H. -P. Liermann and V. Svitlyk and R. Torchio and A. D. Rosa and T. Irifune and A. V. Ponomareva and I. A. Abrikosov and N. Dubrovinskaia and L. Dubrovinsky},
journal= {arXiv preprint arXiv:1905.05497},
year = {2021}
}