Hydrogen (H) is considered to be one of the candidates for light elements in the Earth’s core, but the amount and timing of delivery have been unknown. We investigated the effects of sulfur (S), another candidate element in the core, on deuteration of iron (Fe) in iron–silicate–water system up to 6–12 GPa, ~ 1200 K using in situ neutron diffraction measurements. The sample initially contained saturated water (D2O) as Mg(OD)2 in the ideal composition (Fe–MgSiO3–D2O) of the primitive Earth. In the existence of water and sulfur, phase transitions of Fe, dehydration of Mg(OD)2, and formation of iron sulfide (FeS) and silicates occurred with increasing temperature. The deuterium (D) solubility (x) in iron deuterides (FeDx) increased with temperature and pressure, resulting in a maximum of x = 0.33(4) for the hydrous sample without S at 11.2 GPa and 1067 K. FeS was hardly deuterated until Fe deuteration had completed. The lower D concentrations in the S-containing system do not exceed the miscibility gap (x <  ~ 0.4). Both H and S can be incorporated into solid Fe and other light elements could have dissolved into molten iron hydride and/or FeS during the later process of Earth’s evolution.

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The density of the Earth’s core is several percent lower than that of iron-nickel alloy under conditions of pressure and temperature equivalent to the Earth’s core. Hydrogen is one of the most promising constituents accounting for the density deficit, but hydrogen occupation sites and density decrease of iron-nickel alloy caused by hydrogenation have never been investigated. In this study, the phase relation and crystal structure of Fe0.9Ni0.1Hx(Dx) at high pressures and temperatures up to 12 GPa and 1000 K were clarified by in situ X-ray diffraction and neutron diffraction measurements. Under the P-T conditions of the present study, no deuterium atoms occupied tetragonal (T) sites of face-centered cubic (fcc) Fe0.9Ni0.1Dx, although the T-site occupation was previously reported for fcc FeHx(Dx). The deuterium-induced volume expansion per deuterium vD was determined to be 2.45(4) and 3.31(6) Å3 for fcc and hcp Fe0.9Ni0.1Dx, respectively. These vD values are significantly larger than the corresponding values for FeDx. The vD value for fcc Fe0.9Ni0.1Dx slightly increases with increasing temperature. This study suggests that only 10% of nickel in iron drastically changes the behaviors of hydrogen in metal. Assuming that vD is constant regardless of pressure, the maximum hydrogen content in the Earth’s inner core is estimated to be one to two times the amount of hydrogen in the oceans.

The Earth's core is believed to contain some light elements because it is 10% less dense than pure Fe under the corresponding pressure and temperature conditions. Hydrogen, a promising candidate among light elements, has phase relations and physical properties that have been investigated mainly for the Fe-H system. This study specifically examined an Fe-Si-H system using in-situ neutron diffraction experiments to investigate the site occupancy of deuterium of hcp-Fe 0.95 Si 0.05 hydride at 14.7 GPa and 800 K. To date, this pressure condition is the highest for neutron diffraction experiments conducted at high pressure and high temperature, where crystal structure analysis has been conducted. Results of Rietveld refinement indicate hcp-Fe 0.95 Si 0.05 hydride as having deuterium (D) occupancy of 0.24(2) exclusively at the interstitial octahedral site in the hcp lattice. The effect on the site occupancy of D by addition of 2.6 wt% Si into Fe (Fe 0.95 Si 0.05 ) was negligible compared to results obtained from an earlier study of an Fe-D system (Machida et al., 2019).

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