title: Time-resolved X-ray diffraction and Raman studies of the phase transition mechanisms of methane hydrate.
abstract: The mechanisms by which methane hydrate transforms from an sI to sH structure and from an sH to filled-ice Ih structure were examined using time-resolved X-ray diffractometry (XRD) and Raman spectroscopy in conjunction with charge-coupled device camera observation under fixed pressure conditions. The XRD data obtained for the sI-sH transition at 0.8 GPa revealed an inverse correlation between sI and sH, suggesting that the sI structure is replaced by sH. Meanwhile, the Raman analysis demonstrated that although the 12-hedra of sI are retained, the 14-hedra are replaced sequentially by additional 12-hedra, modified 12-hedra, and 20-hedra cages of sH. With the sH to filled-ice Ih transition at 1.8 GPa, both the XRD and Raman data showed that this occurs through a sudden collapse of the sH structure and subsequent release of solid and fluid methane that is gradually incorporated into the filled-ice Ih to complete its structure. This therefore represents a typical reconstructive transition mechanism.
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
authors.forename: Hisako
authors.initials: H
authors.lastname: Hirai
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Kadobayashi
authors.affiliation: Japan Association of Synchrotron Radiation Institution, Harima 679-5198, Japan.
authors.forename: Naohisa
authors.initials: N
authors.lastname: Hirao
authors.affiliation: Japan Association of Synchrotron Radiation Institution, Harima 679-5198, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Ohishi
authors.affiliation: National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8569, Japan.
authors.forename: Michika
authors.initials: M
authors.lastname: Ohtake
authors.affiliation: National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8569, Japan.
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Yamamoto
authors.affiliation: National Institute for Material Science, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Nakano