Raman study of methane clathrate hydrates under pressure: new evidence for the metastability of structure II
Article first published online: 24 NOV 2006
Copyright © 2006 John Wiley & Sons, Ltd.
Journal of Raman Spectroscopy
Volume 38, Issue 4, pages 440–451, April 2007
How to Cite
Choukroun, M., Morizet, Y. and Grasset, O. (2007), Raman study of methane clathrate hydrates under pressure: new evidence for the metastability of structure II. J. Raman Spectrosc., 38: 440–451. doi: 10.1002/jrs.1665
- Issue published online: 22 MAR 2007
- Article first published online: 24 NOV 2006
- Manuscript Accepted: 27 SEP 2006
- Manuscript Received: 16 MAR 2006
- Raman spectroscopy;
- methane hydrates;
- high-pressure experiments;
- structure II hydrates
New high-pressure experiments on the H2OCH4 system have been conducted for investigating the structure of methane hydrates (MH) under pressure. Interestingly, structure II (sII) MH was generated and locally coexisted with structure I (sI) up to 500 MPa. The Raman analysis of MH formation during the experiments allowed us to distinguish two possible evolutions: (1) a direct crystallization of sI MH from CH4 gas and (2) an indirect evolution from gas to sII, followed by a sII–sI transition. Calculations show that H2O molar fraction is 2.9–25% lower in sII MH than in sI in the early stages of hydrate formation. We suggest that sII MH crystallize locally because the amounts of H2O available for sI MH generation are too low.
A first synthesis of the MH Raman signature under pressure is also provided. In addition to the description of well-known signatures of MH, a new Raman band assigned to a ν3 asymmetric stretching mode of CH bonds in CH4 is observed in sI MH at 3055 cm−1 and at 3060 cm−1 in the high-pressure phase MH II. The first scaling law describing the evolution with pressure of the CH bonds' ν1 peak position within small cages is established. Also, OH and OO bonds' signatures and evolution with pressure are almost identical in MH and ice Ih up to 1 GPa. Such a pressure range is much wider than the stability domain of ice Ih. This result magnifies the role of guest–host interactions in the stabilization of the hydrate structure. Copyright © 2006 John Wiley & Sons, Ltd.