Hydrogen Bond Symmetrization and High-Spin to Low-Spin Transition of ε-FeOOH at the Pressure of Earth’s Lower Mantle

ε-FeOOH may be an important carrier of water into the deep Earth and is a rich system that serves as a model for understanding two widely observed types of pressure-induced phase transitions: hydrogen bond symmetrization, and the high-spin to low-spin transition of the iron cation. We use spin-polarized density functional theory at the PBEsol+U level to investigate structure, energetics, equation of state, phonon dispersion, elasticity, and electronic structure of this phase. We find two distinct phase transtions: H-bond symmetrization at 37 GPa, and the high-spin to low-spin transition at 45 GPa, the latter in excellent agreement with experiment. We find a small, but finite gap typical of semi-conductors that decreases, but does not vanish, with increasing pressure up to at least 100 GPa. Computed vibrational spectra indicate a path towards experimental identification of the hydrogen bond symmetrization transition. Finally, we report our recent extensions to this work that explore hydrogen bond dynamics with molecular dynamics simulations.