Structural evolution of methane hydrate under pressures up to 134 GPa

Abstract

High-pressure experiments were performed to understand the structural evolution of methane hydrate (MH) up to 134 GPa using x-ray powder diffraction (XRD) and Raman spectroscopy with diamond anvil cells. XRD revealed the distinct changes in the diffraction lines of MH owing to phase transition from a guest-ordered state phase [MH-III(GOS)] to a new high-pressure phase (MH-IV) at 33.8–57.7 GPa. MH-IV was found to be stable up to at least 134 GPa without decomposition into solid methane and high-pressure ices. Raman spectroscopy showed the splits in the C–H vibration modes ν3 and ν1 of guest methane molecules in filled-ice Ih (MH-III) at 12.7 GPa and 28.6 GPa, respectively. These splits are caused by orientational ordering of guest methane molecules contained in the hydrate structure, as observed in a previous study. These results suggest that the structural evolution of the filled-ice structure of MH is caused by successive orientational ordering of guest methane molecules, thereby inducing changes in the host framework formed by water molecules.

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

“…The state changes such as phase transition, melting, and solid–solid decomposition of the MH sample can be detected by measuring the frequency of C–H vibration modes of the sample and the pressure in the sample chamber because the C–H vibration modes of methane molecules are very sensitive to the surrounding conditions. [ 10,16,22–26 ] Therefore, in this study, we measured symmetric and antisymmetric stretching vibration modes, ν 1 and ν 3 , respectively, of C–H vibration modes of methane molecules because these two modes show a relatively large pressure dependence, and high‐intensity Raman signals can be obtained in a few minutes under high P–T. [ 10 ]…”

“…The fundamental structure of MH-III is retained up to at least 134 GPa via some phase transitions occurring at 15 GPa and at 34 GPa. [6][7][8][9][10][11] On the basis of its high-pressure stability, MH is thought to be ubiquitous in space, and it is an important constituent of outer icy planets and their moons. Recent experimental and theoretical studies suggested that in Saturn's largest moon Titan, MH exists in the icy mantle as well as in the icy crust, which is an essential source of atmospheric methane.…”

“…The fundamental structure of MH‐III is retained up to at least 134 GPa via some phase transitions occurring at ~15 GPa and at ~34 GPa. [ 6–11 ]…”

“…[ 12,14 ] However, only a few studies have addressed the high P–T stability of MH‐III above 1.8 GPa, and there is a large discrepancy regarding the dissociation conditions between those studies. [ 10,15,16 ] This discrepancy may be caused by the difference in the composition of the experimental systems or by the uncertainty in the evaluation of the dissociation; however, the details are not yet clear. We previously reported that MH‐III dissociates into ice VII and solid methane only through solid–solid decomposition in the relatively higher range above 5 GPa because the dissociation temperature of MH‐III above 5 GPa is significantly lower than the melting temperatures of ice VII and solid methane.…”

“…We previously reported that MH‐III dissociates into ice VII and solid methane only through solid–solid decomposition in the relatively higher range above 5 GPa because the dissociation temperature of MH‐III above 5 GPa is significantly lower than the melting temperatures of ice VII and solid methane. [ 10 ] This result suggests that MH‐III may directly transform into liquid water and fluid methane below 2 GPa because the interpolated dissociation curve of MH‐III intersects the melting curves of both ice VII and solid methane at around 2 GPa. Note that the term “dissociation” in the present paper includes two behaviors, melting and solid–solid decomposition, and does not mean “molecular dissociation.” In addition, although “liquid water” and “fluid methane” could contain a substantial amount of methane and water component, respectively, under high P–T conditions, [ 17 ] we used these terms for simplicity.…”