Anisotropic hydrogen diffusivity in the superionic phase of C–H–O ternary system in ice giants

Hydrogen, oxygen, and carbon are---based on solar system abundances---the main components of ice giants such as Uranus and Neptune, forming the C\ensuremath{-}H\ensuremath{-}O ternary mixture. In the planetary interiors, these materials are subjected to extreme temperature and pressure conditions exceeding 1000 K and 100 GPa. The three elements are assumed to mix in the icy mantles of Uranus and Neptune. However, only the pure C\ensuremath{-}H and ${\mathrm{H}}_{2}\mathrm{O}$ systems have been theoretically and experimentally investigated so far. Here, we identified candidate phase behaviors when the hydrogen, carbon, and oxygen completely mix in extreme conditions relevant to the ice giants by using ab initio molecular dynamics simulations. We reveal that the ternary system undergoes two types of superionicity at similar thermodynamic conditions to the middle icy mantles of Uranus and Neptune. In the first of these superionic phases, the hydrogen atoms diffuse within a fixed sublattice composed of carbon and oxygen. In the second phase, the hydrogen and carbon diffuse simultaneously throughout a fixed oxygen sublattice. Surprisingly, the hydrogen's diffusivity in the superionic phases of the ternary system exhibits an anisotropy, which cannot be seen in the pure superionic water. We further demonstrate the mechanisms of anisotropic hydrogen diffusivity from a microscopic perspective, combining Mulliken population analysis and the Nudged Elastic Band method. These findings provide a material basis for understanding the interior structures, thermal profiles, and enigmatic magnetic fields of the ice giants.