Structural evolution of FeH4 under high pressure

The solid inner core of Earth is mainly composed of an iron-rich alloy with nickel and some lighter elements like hydrogen, carbon, and oxygen, but the exact composition and chemical reactions are still elusive. Hydrogen has been proposed as the main element responsible for the density deficit observed in the Earth's inner core. Moreover, the solubility of hydrogen in iron increases considerably with increasing pressure. Here, we systematically investigated global energetically stable structures of FeH4 in the pressure range of 80-400 GPa using a first-principles structural search. A transition from an insulated alpha-phase to a metallic beta-phase and then to a semi-conductive gamma-phase was predicted. Interestingly, we find a superconducting state in the beta-phase with a transition temperature of 1.70 K at 109.12 GPa. The results are useful for investigating the stable phases and equation of state in the Fe-H system, which are relevant to the Earth's core.