Changes in vibration modes of hydrogen and water molecules and in lattice parameters with pressure for filled-ice hydrogen hydrates

High-pressure experiments regarding two filled-ice hydrogen hydrates, C-1 and C-2, were performed in a pressure range of 0.2-8.0 GPa at room temperature using a diamond anvil cell. According to the first repot of these hydrates by other workers, the hydrogen molecules in C-1 are located interstitially in an ice II lattice and those in C-2 are in an ice Ic lattice. Changes in the Raman frequencies of these hydrates' hydrogen vibrons with pressure were revealed by Raman spectroscopy combined with X-ray diffractometry. Hydrogen molecules in C-1 showed vibration frequencies slightly higher than or similar to those in the fluid at the same pressures, while those in C-2 showed frequencies approximately 20 cm(-1) higher than those in C-1. The OH-stretching mode of the water molecules of C-1 was higher than that of the host ice structures of ice II, while that of C-2 was lower than that of ice VII. This difference in OH-stretching modes between C-1 and C-2 was explained in relation to the intermolecular distances of the water molecules forming each framework structure. Taking these intramolecular vibration modes and the lattice parameter changes into account, the intermolecular interaction between the hydrogen molecules and the host water molecules was discussed.