The Geophysical Properties of FeH_x Phases Under Inner Core Conditions

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.