2-3-2 The transition mechanisms of methane hydrate under high pressure

Abstract

The transition mechanisms of methane hydrate 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 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 a typical reconstructive transition mechanism.