In situ Raman spectroscopic study of the dissociation of methane hydrate at high pressure

The dissociation of methane hydrate at high pressure is studied by in situ Raman spectroscopy in a diamond-anvil cell. As for the Raman spectrum of sI methane hydrate, the nu(1) band of CH4 is split into two peaks v'(1) and v"(1), indicating the partitioning of CH4 between the large (5(12)6(2)) and small (5(12)) cavities, respectively. With increasing temperature, the intensity ratio of I nu'(1)/I nu"(1) decreases obviously, and the d(I nu'(1)/Iv"(1))/dT is -0.079 K-1. Additionally, the nu(1) band of the dissolved CH4 is close to v(1)" of the CH4 molecule engaged in the small 5(12) cavity. This implies that, in the initial stage of hydrate formation, the abundance of small 5(12) cavities is greater than that of large 5(12)6(2) cavities.