High-Temperature Equation of State of FeH: Implications for Hydrogen in Earth's Inner Core

While hydrogen is one of plausible major light elements in the core, the high-temperature equation of state (EoS) of Fe-H alloy has not been experimentally examined to the core pressure range. Here we measured the volume (V) of non-magnetic (NM) face-centered cubic (fcc) FeH at high pressure and temperature (P-T) to 142 GPa and 3660 K in a laser-heated diamond-anvil cell and obtained its P-V-T EoS. An increase in the lattice volume of Fe per H atom, Delta V-H, determined as functions of P and T is found to be substantially smaller than the volume of metallic H that has been used to estimate H concentration in Fe-H alloy. The Delta V-H is almost identical between fcc and double hexagonal close-packed phases in the NM state, suggesting that it may be applicable to hcp. The extrapolation of Delta V-H to inner core conditions indicates its maximum H content to be 0.8-0.9 wt%.