Iron reacts with hydrogen to form solid solutions with body-centered cubic, face-centered cubic, hexagonal close packed, and double hexagonal close packed structures at high temperatures and high pressures. Neutron diffraction is the most powerful tool for determining the occupation sites and occupancies of hydrogen atoms dissolved in a metal lattice. Structural parameters, including hydrogen occupation sites and occupancies, are refined via Rietveld analysis for neutron diffraction data. We present our expertise in Rietveld refinement of iron hydrides accumulated over 10 years.

This article reviews recent progress in the high-pressure studies of iron hydride (FeH_X). The superionic phase transition of FeH_X has recently been predicted from ab initio calculations, and the transition may occur in the Earth's inner core. Approaches for its experimental verification are discussed based on existing studies of picosecond acoustics and electrical resistivity measurements of FeH_X at high pressures generated using a diamond anvil cell. The development of solid-state ionics in high-pressure Earth and planetary sciences is anticipated.

Hydrogen is likely to be one of the important light elements incorporated in the Earth’s core. Melting phase diagram in hydrogen-bearing iron alloy has been an important missing piece to better understand the Earth’s core. Here we review recent melting experiments to determine the melting temperature, liquidus phase relation, and solid-liquid partition coefficient in the Fe-H binary and Fe-Si-H and Fe-O-H ternary systems up to the core pressure range by a combination of in-situ X-ray diffraction measurements and ex-situ textural and chemical characterizations on recovered samples. We also discuss the composition and temperature of the Earth’s core based on these experimental results and recent theoretical calculations.

The Earth’s core contains nonnegligible concentrations of light elements, reducing the density of the iron core. The hydrogen-induced volume expansion of iron and related materials is key to clarify the chemical composition of the core. This article reviews recent advances on the hydrogenation reactions of Si-bearing iron and FeS through in-situ X-ray diffraction and neutron diffraction measurements at high-pressure and high-temperature conditions. The hydrogen-induced volume expansion of hcp Fe_0.95Si_0.05 is 10 % greater than the pure iron and the estimated hydrogen content in the core is about a half of that without the effect of silicon. Stoichiometric FeS (troilite) was hardly hydrogenated, suggesting that the hydrogenation reactions of FeS strongly depend on the stoichiometry of FeS.

Hydrogen is the most abundant element in the solar system and one of the promising candidates of the light elements existing in the Earth’s core. Hydrogen is considered to be supplied from water in early Earth. However, the amount of hydrogen dissolved in the core and its process are still unknown. We have investigated the iron−hydrous mineral system (simulating the ideal composition of primordial Earth) under high pressure and high temperature using in-situ neutron diffraction, X-ray diffraction, and X-ray imaging measurements. We have clarified the mutual interactions among light elements and sulfur’s effect on the hydrogenation of iron during the core−mantle formation process.