Time-resolved X-ray diffraction and Raman studies of the phase transition mechanisms of methane hydrate

Hisako Hirai; Hirokazu Kadobayashi; Naohisa Hirao; Yasuo Ohishi; Michika Ohtake; Yoshitaka Yamamoto; Satoshi Nakano

doi:10.1063/1.4905482

Time-resolved X-ray diffraction and Raman studies of the phase transition mechanisms of methane hydrate

J Chem Phys. 2015 Jan 14;142(2):024707. doi: 10.1063/1.4905482.

Authors

Hisako Hirai¹, Hirokazu Kadobayashi¹, Naohisa Hirao², Yasuo Ohishi², Michika Ohtake³, Yoshitaka Yamamoto³, Satoshi Nakano⁴

Affiliations

¹ Geodynamics Research Center, Ehime University, Matsuyama, Ehime 790-8577, Japan.
² Japan Association of Synchrotron Radiation Institution, Harima 679-5198, Japan.
³ National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8569, Japan.
⁴ National Institute for Material Science, Tsukuba, Ibaraki 305-0044, Japan.

PMID: 25591377
DOI: 10.1063/1.4905482

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.