Observation of methane filled hexagonal ice stable up to 150 GPa

Abstract

Gas hydrates consist of hydrogen-bonded water frameworks enclosing guest gas molecules and have been the focus of intense research for almost 40 y, both for their fundamental role in the understanding of hydrophobic interactions and for gas storage and energy-related applications. The stable structure of methane hydrate above 2 GPa, where CH4 molecules are located within H2O or D2O channels, is referred to as methane hydrate III (MH-III). The stability limit of MH-III and the existence of a new high-pressure phase above 40 to 50 GPa, although recently conjectured, remain unsolved to date. We report evidence for a further high-pressure, room-temperature phase of the CH4–D2O hydrate, based on Raman spectroscopy in diamond anvil cell and ab initio molecular dynamics simulations including nuclear quantum effects. Our results reveal that a methane hydrate IV (MH-IV) structure, where the D2O network is isomorphic with ice Ih, forms at ∼40 GPa and remains stable up to 150 GPa at least. Our proposed MH-IV structure is fully consistent with previous unresolved X-ray diffraction patterns at 55 GPa [T. Tanaka et al., J. Chem. Phys. 139, 104701 (2013)]. The MH-III → MH-IV transition mechanism, as suggested by the simulations, is complex. The MH-IV structure, where methane molecules intercalate the tetrahedral network of hexagonal ice, represents the highest-pressure gas hydrate known up to now. Repulsive interactions between methane and water dominate at the very high pressure probed here and the tetrahedral topology outperforms other possible arrangements in terms of space filling.

Water and methane stay together at extreme pressures

“…The evidence for these discoveries is based on Raman spectroscopy carried out in a diamond anvil cell and supported by high-level computational calculations. Schaack et al (8) also show that their proposed structure for MH-IV is consistent with previously unresolved X-ray diffraction data reported by Tanaka et al (12). The mechanism of the MH-III to MH-IV phase transition is quite subtle and requires the reorganization of only a few hydrogen bonds, which explains why the H 2 O:CH 4 ratio can remain constant at 2:1 during the phase transition.…”

“…Just as for MH-III (11), Schaack et al report that MH-IV eventually undergoes hydrogen-bond symmetrization upon compression. Overall, the MH-IV structure was found to be stable up to at least 150 GPa, the current limit of their experiments (8).…”

“…However, this is not where the story ends. Pushing toward even higher pressures, Schaack et al (8) now report that yet another methane-filled ice structure, which they name MH-IV, exists above 40 GPa. In contrast to MH-III, the water network of MH-IV is simple, containing only 6-membered rings in a structure that is very similar to the "ordinary" ice Ih as shown in Fig.…”

“…This is significant since it was originally suspected that MH decomposes into pure ice and methane at pressure as low as ∼1 GPa (13). The remarkable pressure stability of MH-IV therefore opens up an exciting opportunity for studying the interactions between H 2 O and CH 4 over a wide pressure window, and its existence is of course also highly relevant for understanding planetary processes involving water and methane, for example inside gas giants such as Uranus and Neptune (8).…”

Quantum driven proton diffusion in brucite-like minerals under high pressure

“…As in other hydrous minerals 24,25 , hydrates 36 and ices 15,37 , the proton can hop along hydrogen bonds. Following Dupuis et al 28 , we refer to this process here as dissociation, as it implies the breaking of a covalent O-H bond to form another distinct O-H covalent bond.…”