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

Area of Science:

• Materials Science
• Physical Chemistry
• Solid State Physics

Background:

• Hydrogen hydrates are investigated for hydrogen storage due to reversible H2 uptake.
• C2 hydrogen hydrate forms a cubic ice (ice Ic) framework and exists at high pressures (above 2 GPa).
• The discovery of plastic ice VII motivates research into plastic phases of other hydrates.

Purpose of the Study:

• To investigate the potential for a plastic phase in C2 hydrogen hydrate.
• To refine computational models for accurate phase diagram prediction of hydrogen hydrates.
• To understand the conditions necessary for first-order phase transitions in crystalline solids.

Main Methods:

• Refinement of an all-atom potential model for hydrogen hydrates.
• Phase diagram calculations for C2 hydrogen hydrate.
• Analysis of water rotational dynamics and hydrogen atom distributions via simulation.

Main Results:

• The refined model accurately reproduces the C2 hydrogen hydrate phase diagram.
• Enhanced water rotational dynamics were observed at elevated temperatures, but no first-order phase transition occurred.
• Hydrogen atoms in C2 hydrate and ice VII occupy tetrahedral sites, unlike plastic ice VII where they occupy cubic vertices.

Conclusions:

• C2 hydrogen hydrate lacks the structural characteristics for a plastic phase due to its single-lattice nature, preventing local symmetry changes except via melting.
• Rapid rotational motion of water molecules is insufficient for forming a distinct plastic phase.
• The ability to undergo a local symmetry change is the critical factor determining a first-order phase transition in crystalline hydrates.