This study is led by Prof. Hanyu Liu, Prof. Yanming Ma at the College of Physics, Jilin University located in Changchun, China, and Prof. Changfeng, Chen at the Department of Physics and Astronomy, University of Nevada. Their supercomputer simulations present evidence of direct and prevalent chemical association of hydrogen and helium facilitated by their reaction with recently discovered iron peroxide FeO₂ in forming rare quaternary compound FeO₂H₂He, which has been predicted to be viable in a large region of the pressure-temperature phase diagram. Most interestingly, in a wide swath of the phase space corresponding to Earth's lowest-mantle regions, this quaternary compound stays in a superionic state hosting liquid-like hydrogen inside the FeO₂He sublattice that remains intact in crystalline form. This exotic solid-liquid mixture state of matter makes an unusually conducive environment promoting close coalescence of hydrogen and helium. “These results highlight a compelling case of a hydrogen-helium chemical association, which may be harbored in deep-Earth regions, providing crucial guidance for exploring the novel solid and superionic phases of FeO₂H₂He in laboratory experiments and also for modeling interiors of giant solar and extrasolar planets,” Liu says. 

Hydrogen and helium are the most abundant elements in the universe and play crucial roles in geological and astrophysical environments, but they are known to be inert toward each other across wide pressure-temperature and concentration ranges and remain largely immiscible up to multi-megabar pressures and 3,000-4,000 K temperatures. Given their prominent presence and influence on the formation and evolution of celestial bodies, it is of great interest and significance to explore and decipher the nature of interactions between hydrogen and helium, the especially possible chemical association that would have considerable impacts in many scientific fields, from chemistry, physics, geoscience to astrophysics.

By employing an advanced crystal structure search method called CALYPSO, they identified a quaternary compound FeO₂H₂He, which could be stabilized in a wide range of pressure and temperature conditions. Further molecular dynamics simulations indicate a novel superionic state of FeO₂H₂H, which hosted liquid-like diffusive hydrogen in the FeO₂He sublattice. Meanwhile, a conducive environment for hydrogen-helium chemical association was identified, where the pressure and temperature conditions correspond to the Earth's lowest mantle regions. “These results suggested the surprising chemically facilitated coalescence of otherwise immiscible molecular species highlights a promising avenue for exploring this long-sought but [a] hitherto unattainable state of matter”, Liu says, “this finding raises strong prospects for exotic H-He mixtures inside Earth, as well as providing implications for other astronomical bodies.