Unraveling the Structure and Bonding Evolution of the Newly Discovered Iron Oxide FeO2

Recently reported synthesis of ${\mathrm{FeO}}_{2}$ at high pressure has stimulated great interest in exploring this new iron oxide and elucidating its properties. Here, we present a systematic computational study of crystal structure, chemical bonding, and sound velocity of ${\mathrm{FeO}}_{2}$ in a wide range of pressure. Our results establish thermodynamic stability of the experimentally observed pyrite phase (P-phase) of ${\mathrm{FeO}}_{2}$ at pressures above 74 GPa and unveil two metastable ${\mathrm{FeO}}_{2}$ phases in $Pbcn$ and $P{4}_{2}/mnm$ symmetry at lower pressures. Simulated x-ray diffraction (XRD) spectra of $Pbcn$ and $P{4}_{2}/mnm\phantom{\rule{4pt}{0ex}}{\mathrm{FeO}}_{2}$ match well with measured XRD data of the decompression products of P-phase ${\mathrm{FeO}}_{2}$, providing compelling evidence for the presence of these metastable phases. Energetic calculations reveal unusually soft O-O bonds in P-phase ${\mathrm{FeO}}_{2}$ stemming from a low-frequency libration mode of ${\mathrm{FeO}}_{6}$ octahedra, rendering the O-O bond length highly sensitive to computational and physical environments. Calculated sound-velocity profiles of P-phase ${\mathrm{FeO}}_{2}$ are markedly different from those of the $Pbcn$ and $P{4}_{2}/mnm$ phases, underscoring their distinct seismic signatures. Our findings offer insights for understanding the rich structural, bonding, and elastic behaviors of this newly discovered iron oxide.