The Geophysical Properties of FeHx Phases Under Inner Core Conditions

Abstract Hydrogen has been proposed as an important light element in planetary iron cores, while the crystal structure and thermoelasticity of FeHx (x = 1) (FeH hereafter) under inner core conditions remain largely unknown. Recent studies report that FeH adopts an face‐centered cubic (fcc) structure up to core conditions. In this study, using ab initio molecular dynamics, we calculate the free energy and elastic properties of FeH at high P‐T conditions. Our results indicate that the hexagonal close‐packed (hcp) structure of FeHx is favored by both the low hydrogen concentration and the elevated temperature of inner‐core conditions. We also clarify that lattice hydrogen hardens the wave velocities of iron while superionic hydrogen softens it. Both fcc‐ and hcp‐FeH can match inner‐core wave velocities and Poisson's ratio, which supports the hypothesis of hydrogen as a vital light element in the Earth's core.