Hydrogen bond symmetrization and high-spin to low-spin transition of -FeOOH at the pressure of Earth's lower mantle

We focus on the ferric end-member of phase H: epsilon-FeOOH using density functional theory at the PBEsol+U level. At 300 K, we find that epsilon-FeOOH undergoes a hydrogen bond symmetrization at 37 GPa and a sharp high-spin to low-spin transition at 45 GPa. We find excellent agreement with experimental measurements of the equation of state, lattice parameters, atomic positions, vibrational frequencies, and optical properties as related to the band gap, which we find to be finite and small, decreasing with pressure. The hydrogen bond symmetrization transition is neither first- nor second-order, with no discontinuity in volume or any of the elastic moduli. Computed IR and Raman frequencies and intensities show that vibrational spectroscopy may provide the best opportunity for locating the hydrogen bond symmetrization transition experimentally. We find that epsilon-FeOOH is highly anisotropic in both longitudinal- and shear-wave velocities at all pressures, with the shear wave velocity varying with propagation and polarization direction by as much as 24% at zero pressure and 43% at 46 GPa. The shear and bulk elastic moduli increase by 18% across the high-spin to low-spin transition.