Stability of hydrogen-filled hexagonal ice under high pressure

Abstract

Hydrogen hydrates, composed of a water (H2O) host framework enclosing hydrogen (H2) molecules, exhibit diverse crystal structures. The stability of their high-pressure phases provides fundamental insight into the constituents of giant planets—rich in both H2O and H2—as well as their potential for energy storage on Earth. While H2-filled ice Ic (C2 hydrate) is known to be stable above 2 GPa, the possible existence of H2-filled ice Ih (named CIh hydrate) has remained largely unexplored. Here, we employ hybrid grand-canonical Monte Carlo and isothermal–isobaric molecular dynamics simulations to demonstrate that ice Ih can absorb H2 up to an H2-to-H2O ratio of 0.5 upon compression. Moreover, free energy calculations reveal that CIh hydrate becomes thermodynamically stable under a few gigapascals, thereby occupying the stability conditions previously attributed to C1 hydrate.