Raman study of methane clathrate hydrates under pressure: new evidence for the metastability of structure II

New high-pressure experiments on the (HO)-O-2-CH4 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 nu(3) asymmetric stretching mode of C-H 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 C-H bonds' nu(1) peak position within small cages is established. Also, O-H and O-O 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 (c) 2006 John Wiley & Sons, Ltd.