Pressure-induced phase transitions in clathrate hydrates

Ice I transforms to a high-density amorphous phase when pressed to 10 kbar at 77 K. Similar transformations in structure I and structure II clathrate hydrates have been studied by pressing samples in a piston-cylinder apparatus and by molecular dynamics simulations. The simulations were also carried out on structure I and II empty lattices. The hydrates and the empty lattices were found to transform to high-density phases under pressure at 77 K. The high-density phases of the empty lattices could be recovered at zero pressure, as is possible in the case of high-density amorphous phase of ice. However, it was not possible to recover the high-density phases of the hydrates at zero pressure. Instead, they reverted back to their original crystalline structures when the pressure was released. The molecular dynamics results suggest that under pressure the water molecules in the hydrates collapse around the guest molecules, and the repulsive forces between the guest and the water molecules are mainly responsible for the reversible transition to the original structure when the pressure is released.