Stability of hydrogen-filled hexagonal ice under high pressure

QL Xue and K Mochizuki, JOURNAL OF CHEMICAL PHYSICS, 163, 234702 (2025).

DOI: 10.1063/5.0303895

Hydrogen hydrates, composed of a water (H2O) host framework enclosing hydrogen (H-2) 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 H-2-as well as their potential for energy storage on Earth. While H-2-filled ice Ic (C-2 hydrate) is known to be stable above 2 GPa, the possible existence of H-2-filled ice Ih (named C-Ih 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 H-2 up to an H-2-to-H2O ratio of 0.5 upon compression. Moreover, free energy calculations reveal that C-Ih hydrate becomes thermodynamically stable under a few gigapascals, thereby occupying the stability conditions previously attributed to C-1 hydrate.

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