Helium-hydrogen immiscibility at high pressures

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

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.

Ab initio study of the miscibility for solid hydrogen-helium mixtures at high pressure

Understanding the behavior of H₂-He binary mixtures at high pressure is of great importance. Two more recent experiments [J. Lim and C. S. Yoo, Phys. Rev. Lett. 120, 165301 (2018) and R. Turnbull et al., ibid. 121, 195702 (2018)] are in conflict, regarding the miscibility between H₂ and He in solids at high pressure. On the basis of first-principles calculations combined with the structure prediction method, we investigate the miscibility for solid H₂-He mixtures at pressures from 0 GPa to 200 GPa. It is found that there is no sign of miscibility and chemical reactivity in H₂-He mixtures with any H:He ratio. Moreover, instead of H₂-He mixtures, the calculated Raman modes of the N-H mixtures can better explain the characteristic peaks observed experimentally, which were claimed to be the H-He vibrational modes. These calculation results are more in line with the experimental findings by Turnbull et al. [Phys. Rev. Lett. 121, 195702 (2018)].