Superstoichiometric Alloying of H and Close-Packed Fe-Ni Metal Under High Pressures: Implications for Hydrogen Storage in Planetary Core

Although high pressure enables alloying between hydrogen and iron, hydrogen-to-iron molar ratio (H/Fe) so far found in experiments is mostly limited to 1 in the close-packed iron metal under high pressure. We report a H/(Fe + Ni) ratio of 1.8 +/- 0.1 from (Fe,Ni)H-x (or x >= 1.8) quenched from liquid, exceeding the amounts so far reported for densely packed Fe alloys. From the metastable behavior of the frozen (Fe,Ni)H-x liquid during decompression, we infer that the amount is a lower bound and therefore even a greater amount of H can be dissolved in the liquid part of Fe-rich cores of planets. The significant H storage capacity of liquid Fe-Ni alloy is important to consider for potential storage of H in the interiors of low-density planets as well as rocky planets.