Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides,resulting in numerous compounds forming homologous seriesnFeOmFe2O3and the appearance of FeO2.Here, based on the results ofin situsingle-crystal x-ray diffraction, Mössbauer spectroscopy, x-rayabsorption spectroscopy, and density-functional theoryþdynamical mean-field theory calculations, wedemonstrate that iron in high-pressure cubic FeO2and isostructural FeO2H0.5is ferric (Fe3þ), and oxygenhas a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 canbe explained by a formation of a localized hole at oxygen sites.

Koemets, E. and Leonov, I. and Bykov, M. and Bykova, E. and Chariton, S. and Aprilis, G. and Fedotenko, T. and Cl\’ement, S. and Rouquette, J. and Haines, J. and Cerantola, V. and Glazyrin, K. and McCammon, C. and Prakapenka, V. B. and Hanfland, M. and Liermann, H.-P. and Svitlyk, V. and Torchio, R. and Rosa, A. D. and Irifune, T. and Ponomareva, A. V. and Abrikosov, I. A. and Dubrovinskaia, N. and Dubrovinsky, L., “Revealing the Complex Nature of Bonding in the Binary High-Pressure Compound FeO2”, Phys. Rev. Lett. 126, 106001, DOI: 10.1103/PhysRevLett.126.106001 abstract

a) Crystal structure and valence electron density plotfor HP-PdF2FeO2obtained by DFTþDMFT at∼70GPa.(b) DFT hybride potential Heyd–Scuseria–Ernzerhof HSE03results for pyrite-type MgO2peroxide at ambient conditions.Max stands for 15% of maximum of charge densityρðrÞ