Picosecond-Scale Water Rotation in C₂ Hydrogen Hydrate Achieved through a Continuous Change

Hydrogen hydrates have attracted attention as potential energy storage materials due to their ability to accommodate and release large amounts of hydrogen gas (H₂) upon freezing and melting of the water (H₂O) sublattice. C₂ hydrogen hydrate, which consists of a cubic ice (ice Ic) framework, occupies a large region above 2 GPa in the phase diagram, similar to that of ice VII in pure water. The recent experimental discovery of plastic ice VII has prompted the search for a plastic form of the C₂ hydrate. We begin by refining the existing all-atom potential model to accurately reproduce the phase diagram of hydrogen hydrate. Our findings show that while the rotational dynamics of water in C₂ hydrate are significantly enhanced at elevated temperatures, there is no evidence of a first-order phase transition. Analysis of hydrogen distributions reveals that in C₂ hydrate and ice VII, hydrogen atoms jump between tetrahedral sites, whereas in plastic ice VII, they occupy cubic vertices. Due to the single-lattice nature of C₂ hydrate, it cannot undergo a local symmetry change except through melting into the liquid phase. We suggest that rapid rotational motion alone is not a sufficient condition for the formation of a distinct plastic phase. Instead, the key determinant of a first-order phase transition is the ability to undergo a local symmetry change.