A Clathrate Hydrate Structure Hidden in Plain Sight.

Area of Science:

• Materials Science
• Physical Chemistry
• Crystallography

Background:

• Clathrate hydrates exhibit diverse crystal structures, with sI, sII, and sH being the most common.
• Other clathrate structures are typically considered only in the presence of specific guest molecules.
• The prevalence and formation mechanisms of less common clathrate structures remain underexplored.

Purpose of the Study:

• To report the observation of a previously unrecognized clathrate hydrate structure, the tetragonal structure (TS-I).
• To investigate the formation conditions and implications of the TS-I structure in gas hydrates.
• To assess the potential role of TS-I in the coexistence and transitions between common clathrate structures.

Main Methods:

• Molecular dynamics simulations were employed to observe and analyze the formation of the tetragonal structure (TS-I).
• Simulations focused on scenarios involving the contact of sI crystal grains with differing growth orientations.
• Analysis also considered cases of geometrical frustration during the growth of sI crystals.

Main Results:

• The tetragonal structure (TS-I) was observed as a cocrystal under specific conditions of sI crystal growth.
• TS-I formation was linked to the contact between sI crystal grains with different growth directions.
• Evidence suggests TS-I plays a role in the combination and/or transition between sI and sII clathrate structures.

Conclusions:

• A hidden tetragonal structure (TS-I) can form in commonly occurring gas hydrates.
• TS-I may be more prevalent than previously thought, potentially existing whenever sI or sII structures are formed.
• Further experimental and simulation-based identification of TS-I is crucial for a comprehensive understanding of clathrate hydrates.