Noble-Gas Solubility in Solid and Fluid Metallic Hydrogen

TL;DR

This study examines noble-gas solubility in metallic hydrogen at high pressure, revealing phase-dependent behavior and implications for giant-planet interior composition.

Contribution

It provides the first ab initio analysis of noble-gas impurities in metallic hydrogen, highlighting phase-specific solubility and fractionation mechanisms.

Findings

He and Ne phase separate in liquid hydrogen.

Ar, Kr, Xe are soluble in liquid hydrogen.

All noble gases have positive formation free energies in solid metallic hydrogen.

Abstract

Metallic hydrogen dominates the deep interiors of giant planets, where trace elements interact with dense quantum matter under extreme pressure. We investigate the thermodynamic stability of noble-gas impurities (He, Ne, Ar, Kr, Xe) in metallic hydrogen at 500 GPa using ab initio molecular dynamics combined with first-principles free-energy calculations. In the solid metallic phase, all noble gases exhibit positive formation free energies, driven by unfavorable electronic enthalpy and zero-point vibrational contributions. By contrast, heavier noble gases (Ar, Kr, Xe) appear soluble in liquid hydrogen, while He and Ne phase separate. This crossover reflects a competition between electronic repulsion and disorder-driven stabilization intrinsic to the liquid phase. Our results reveal noble-gas retention in metallic hydrogen, providing a microscopic mechanism for noble-gas fractionation in giant-planet interiors.