Hydrogen‐Induced Volume Expansion in Hexagonal Close‐Packed Iron: Effects of Pressure, Temperature, and Crystal Structure

Abstract

Hydrogen is a promising candidate for the “light” element in terrestrial planetary cores. Its incorporation into iron causes significant volume expansion, leading to a substantial density deficit. Although extensive studies have been conducted on iron hydride (FeH _x ) with the fcc structure, the thermoelastic properties on FeH _x with the hcp structure (hcp‐FeH _x ) remain unconstrained because of the experimental difficulties to control hydrogen content. Here, we synthesized hcp‐FeH _x with controlled hydrogen contents under high‐pressure and high‐temperature conditions. We carried out in situ X‐ray diffraction measurements on hcp‐FeH _x at 10–25 GPa and 300–900 K using a Kawai‐type multianvil apparatus and constructed their equations of state. By combining the equation of state (EoS) of hcp‐FeH _x determined in the present study with hydrogen contents independently constrained by prior neutron diffraction experiments and EoS models for hcp‐Fe from the literature, we demonstrated that the discrepancies in the hydrogen‐induced volume expansion coefficient can be clearly explained by its pressure and temperature dependence. Our results revealed that the hydrogen‐induced volume expansion of hcp‐Fe exhibits a strong temperature dependence at low pressures, but its temperature effect significantly weakens with increasing pressure. We also showed that the density reduction of Fe by hydrogenation depends on its crystal structure. These findings demonstrate that estimates of hydrogen content in iron at planetary interior conditions based on hydrogen‐induced volume expansion need to be revised by properly accounting for its PT ‐dependence and crystal structure.