Novel Hydrogen Clathrate Hydrate.

Area of Science:

• Materials Science
• Chemistry
• Physics

Background:

• Clathrate hydrates are crystalline solids where water molecules form cages trapping guest molecules.
• Hydrogen clathrate hydrates are of particular interest due to their potential applications and unique structural properties.
• Previous studies have focused on ordered structures of hydrogen clathrate hydrates.

Purpose of the Study:

• To synthesize and characterize a new hydrogen clathrate hydrate phase.
• To investigate the structural and dynamic properties of this novel clathrate.
• To compare the new phase with known ordered clathrate structures and ice polymorphs.

Main Methods:

• Synthesis of hydrogen clathrate hydrate at high pressure (1.2 GPa) and ambient temperature (298 K).
• Characterization using single-crystal X-ray diffraction and Raman spectroscopy.
• Theoretical investigations employing first-principles calculations.

Main Results:

• A new hydrogen clathrate hydrate with a trigonal space group (R3c or R3[over ¯]c) was successfully synthesized.
• The oxygen sublattice matches that of ice II, with hydrogen molecules occupying ring cavities, yielding the composition (H_{2}O)_{6}H_{2}.
• Raman spectroscopy and theoretical calculations confirmed a hydrogen-disordered nature for the new phase (C_{1}^{'}).

Conclusions:

• The newly synthesized clathrate hydrate represents a disordered form of ice II, previously unobserved.
• This disordered phase transitions to the ordered C_{1} clathrate under compression and/or cooling.
• The findings provide new insights into the structural diversity and phase behavior of hydrogen clathrate hydrates.