Structural changes and intermolecular interactions of filled ice Ic structure for hydrogen hydrate under high pressure

Abstract

High-pressure experiments of hydrogen hydrate were performed using a diamond anvil cell under conditions of 0.1–44.2 GPa and at room temperature. Also, high pressure Raman studies of solid hydrogen were performed in the pressure range of 0.1–43.7 GPa. X-ray diffractometry (XRD) for hydrogen hydrate revealed that a known high-pressure structure, filled ice Ic structure, of hydrogen hydrate transformed to a new high-pressure structure at approximately 35–40 GPa. A comparison of the Raman spectroscopy of a vibron for hydrogen molecules between hydrogen hydrate and solid hydrogen revealed that the extraction of hydrogen molecules from hydrogen hydrate occurred above 20 GPa. Also, the Raman spectra of a roton revealed that the rotation of hydrogen molecules in hydrogen hydrate was suppressed at around 20 GPa and that the rotation recovered under higher pressure. These results indicated that remarkable intermolecular interactions in hydrogen hydrate between neighboring hydrogen molecules and between guest hydrogen molecules and host water molecules might occur. These intermolecular interactions could produce the stability of hydrogen hydrate.