Sequential in situ Raman spectroscopy for observing dissociation behavior of filled-ice Ih of methane hydrate at high pressure

In situ Raman spectroscopy was performed to investigate the stability and dissociation behavior of filled-ice Ih of methane hydrate at high-pressure (up to 5.1 GPa) and high-temperature (up to 408 K) using an externally heated diamond anvil cell. The results revealed that filled-ice Ih was stable up to 383 K at 4.5 GPa, and the dissociation conditions intersected with the melting curve of ice VII at approximately 2.5 GPa and with that of solid methane at approximately 2.2 GPa. Consequently, the dissociation into water and methane components occurred via two different mechanisms depending on pressure: (1) melting into liquid water and fluid methane below approximately 2.5 GPa and (2) solid–solid decomposition into ice VII and solid methane above approximately 2.5 GPa. Visual observations with an optical microscope synchronized with in situ Raman spectroscopy detected considerable changes in the sample in the case of melting, whereas they could not detect any changes in the decomposition case because both decomposition products were solid phases with similar refractive indices. This result demonstrated that the synchronized and sequential Raman spectroscopy with optical observations is indispensable for evaluating the dissociation behavior; otherwise, the solid–solid decomposition is overlooked when the evaluation is performed using a microscope only. The stability field of filled-ice Ih obtained in the experiment can help infer the states of methane hydrates in the interior of icy bodies such as Saturn's largest moon, Titan.