Ionization of H in iron oxy-hydroxide

The symmetrization of O-H bonds in hydroxide often induces a slightly first order phase transition. However, the phase transition causes dramatic reconfiguring of electronic properties. In this work, we pressurized a piece of iron oxy-hydroxide (FeO₂H) up to 60 GPa. Upon the asymmetric O-H bonds symmetrize to O-H-O, the measured electric conductivity surged a few orders of magnitude higher. Combining with first-principle simulation, the symmetrization separates H⁺ ion from the hydroxyl OH^-1 group by forming ionic-type double O-H bonds. Although the pressure range of symmetrization coincides with the high-low spin transition of Fe, the sudden boost of electric conductivity is against conventional spin-transition effect, which reduces conductivity. Therefore, the ionic phase transition of H is likely to be responsible for the change of conductivity. The ionization of H is among the many intriguing behaviors of H under extreme pressure conditions and may have profound influences for the fundamental processes in Earth’s deep interiors.