High-pressure Raman and Brillouin studies of single crystalline gas hydrates

• Published in: Canadian Journal of Physics
• Main Author: Shimizu, H
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2003
• Subjects: General Physics and Astronomy; Anvil; Methane hydrate
• Online Access: http://dx.doi.org/10.1139/p03-026; http://www.nrcresearchpress.com/doi/pdf/10.1139/p03-026

Methane hydrate (MH) and argon hydrate (AH) single crystals were synthesized in a diamond anvil cell to investigate their intrinsic high-pressure properties by visual observation and in situ Raman and Brillouin-scattering measurements. Single crystalline MH shows clearly two phase transitions at P =0.9 and 1.9 GPa in its crystalline shape and state under a microscope, and in the change in its Raman spectra of C–H stretching vibrations of guest CH 4 molecules. The first determination of the elastic properties in MH sI phase has revealed that MH is about 10% softer than the most common ice Ih, which is accounted for by MH's void-rich and open structures. For a single-crystal AH, we have observed two phase transitions from AH-I (sII) to AH-II at about P =0.65 GPa and from AH-II to AH-III at P =1.02 GPa. The breakdown of the cage structure in AH-III was confirmed by the disappearance of the O–H stretching mode of host H 2 O lattices. The possibility of Ar double occupancy in the large cages of sII and higher pressure phases is investigated by the low-frequency Raman peak observed around 120 ~ 140 cm –1 in view of recent MD calculations. PACS Nos.: 62.30+d, 62.50+p, 62.65+k, 64.70-p, 78.30-j, 78.35+c