What is the form of H in the Earth's inner core?

Previous work on the possible presence of H in the Earth's inner corefocuses on FeH as the thermodynamically stable form of Fe-H compounds at the pressures of the Earth's core.¹ In the present work we have carried out a systematic structure search of the Fe-H compounds at high pressure using a particle swarm optimization algorithm, as implemented in CALYPSO. Each structure has then been studied through ab initio calculations within the framework of the density functional theory, using VASP, and we have obtained the convex hull of the thermodynamically stable compounds. We have found that the Fe-rich compound Fe₅H becomes thermodynamically stable at high pressure. This compound is structurally very different from FeH, but similar to the compounds Fe_2-4H: it consists of an hcp-Fe lattice with H atoms arranged regularly in the octahedral interstices.

[1] Bazhanova, Z. G. et al., Physics-Uspekhi, 55, 489 (2012)