Direct H-He chemical association in superionic FeO2H2He at deep-Earth conditions

Prediction evidenced a chemical association of hydrogen and helium facilitated by their reaction with iron peroxide in forming a rare quaternary compound FeO2H2He, raising strong prospects for exotic H-He mixtures inside Earth and possibly also in other astronomical bodies. Hydrogen and helium are known to play crucial roles in geological and astrophysical environments; however, they are inert toward each other across wide pressure-temperature (P-T) conditions. Given their prominent presence and influence on the formation and evolution of celestial bodies, it is of fundamental interest to explore the nature of interactions between hydrogen and helium. Using an advanced crystal structure search method, we have identified a quaternary compound FeO2H2He stabilized in a wide range of P-T conditions. Ab initio molecular dynamics simulations further reveal a novel superionic state of FeO2H2He hosting liquid-like diffusive hydrogen in the FeO2He sublattice, creating a conducive environment for H-He chemical association, at P-T conditions corresponding to the Earth's lowest mantle regions. To our surprise, this chemically facilitated coalescence of otherwise immiscible molecular species highlights a promising avenue for exploring this long-sought but hitherto unattainable state of matter. This finding raises strong prospects for exotic H-He mixtures inside Earth and possibly also in other astronomical bodies.

Automated summary

A new quaternary compound FeO2H2He has been identified, stabilized at high-pressure and high-temperature conditions, providing new theoretical evidence for the existence of exotic H-He mixtures inside Earth and other astronomical bodies.