3.2 Thermodynamic Aspects of Gas Hydrates
71
maintained Structure I. At 2.1 GPa, coexisting ice VI transformed to ice VII and the
fluid methane solidified to phase I, while Structure I of methane hydrate was still
maintained. At 2.3 GPa, all of the remaining sI methane hydrate decomposed into
ice VII and phase I of solid methane [54].
About a year later, Chou et al. reported that sI methane hydrate did not decompose into ice VII and phase I of solid methane, as earlier thought [54], but instead
transformed into two new structures of methane hydrate at high pressures [55]. One
of such structures was sH-like hexagonal and the other sII-like cubic, both different
from the sI methane hydrate structure at low pressures [55].
Loveday et al. then elucidated the two-phase transitions in methane hydrate at
ultrahigh pressures [56]. The first was from the commonly known sI to a new methane
hydrate phase (MH-II) between 0.8 and 1.1 GPa [56]. The MH-II phase then underwent a transformation at 2.0 GPa to another new methane hydrate phase of III (MHIII) which remained stable up to 10 GPa [56]. Structurally, MH-II was found to
be hexagonal and structurally similar to sH [57], whereas MH-III was structurally
similar to common ice (Ih) [58].
At about the same time, Manakov et al. reported phase transformations of argon
hydrate at high pressures and room temperature [59, 60]. Three high-pressure phase
transitions were observed: from sII to the high-pressure hexagonal structure at 0.46
GPa, from the hexagonal to a tetragonal structure at 0.77 GPa and from the tetragonal
structure to ice (II) at 0.96 GPa [59, 60]. Shimizu et al. then reported phase transformations of single-crystalline argon hydrate at high pressures and room temperature
[61]. Single-crystalline argon hydrate at room temperature showed three phase transitions from sII to a high-pressure hexagonal structure at 0.43 GPa, from the hexagonal
to a tetragonal structure at 0.66 GPa and from the tetragonal structure to ice at 1.05
GPa [61].
Loveday et al. later extended their earlier ultrahigh-pressure studies to other sI
and sII hydrates at room temperature [62]. Methane, argon, nitrogen, and xenon
hydrates all transformed into hexagonal (MH-II type) structure with increasing pressure. Methane, argon, and nitrogen hexagonal hydrate then transformed into an
orthorhombic hydrate with further increase in pressure, whereas xenon hexagonal
hydrate decomposed to ice and xenon around ≈2.5 GPa. This decomposition of
xenon hydrate appears similar to the decomposition of methane hydrate at 2.3 GPa
reported by Hirai et al. earlier [54]. So, decomposition of at least some clathrate
hydrates at high pressures appears real. If the water host can no longer maintain
hollow structures that are large enough to accommodate guest molecules due to the
very high pressures, then the guest “solubility” in the solid water host would go down
and the clathrate hydrate would lose its source of thermodynamic stability.
Meanwhile, Yang et al. reported a new structure of xenon hydrate under atmospheric pressure at a cryogenic temperature [63]. The new phase was derived by
initial pressurization of sI xenon hydrate to 2.0 GPa at room temperature followed
by quenching to 77 K at atmospheric pressure. The new structure has a hexagonal
symmetry that is similar to MH-II and remains stable up to 160 K before decomposing
to the traditional sI phase [63].
71
maintained Structure I. At 2.1 GPa, coexisting ice VI transformed to ice VII and the
fluid methane solidified to phase I, while Structure I of methane hydrate was still
maintained. At 2.3 GPa, all of the remaining sI methane hydrate decomposed into
ice VII and phase I of solid methane [54].
About a year later, Chou et al. reported that sI methane hydrate did not decompose into ice VII and phase I of solid methane, as earlier thought [54], but instead
transformed into two new structures of methane hydrate at high pressures [55]. One
of such structures was sH-like hexagonal and the other sII-like cubic, both different
from the sI methane hydrate structure at low pressures [55].
Loveday et al. then elucidated the two-phase transitions in methane hydrate at
ultrahigh pressures [56]. The first was from the commonly known sI to a new methane
hydrate phase (MH-II) between 0.8 and 1.1 GPa [56]. The MH-II phase then underwent a transformation at 2.0 GPa to another new methane hydrate phase of III (MHIII) which remained stable up to 10 GPa [56]. Structurally, MH-II was found to
be hexagonal and structurally similar to sH [57], whereas MH-III was structurally
similar to common ice (Ih) [58].
At about the same time, Manakov et al. reported phase transformations of argon
hydrate at high pressures and room temperature [59, 60]. Three high-pressure phase
transitions were observed: from sII to the high-pressure hexagonal structure at 0.46
GPa, from the hexagonal to a tetragonal structure at 0.77 GPa and from the tetragonal
structure to ice (II) at 0.96 GPa [59, 60]. Shimizu et al. then reported phase transformations of single-crystalline argon hydrate at high pressures and room temperature
[61]. Single-crystalline argon hydrate at room temperature showed three phase transitions from sII to a high-pressure hexagonal structure at 0.43 GPa, from the hexagonal
to a tetragonal structure at 0.66 GPa and from the tetragonal structure to ice at 1.05
GPa [61].
Loveday et al. later extended their earlier ultrahigh-pressure studies to other sI
and sII hydrates at room temperature [62]. Methane, argon, nitrogen, and xenon
hydrates all transformed into hexagonal (MH-II type) structure with increasing pressure. Methane, argon, and nitrogen hexagonal hydrate then transformed into an
orthorhombic hydrate with further increase in pressure, whereas xenon hexagonal
hydrate decomposed to ice and xenon around ≈2.5 GPa. This decomposition of
xenon hydrate appears similar to the decomposition of methane hydrate at 2.3 GPa
reported by Hirai et al. earlier [54]. So, decomposition of at least some clathrate
hydrates at high pressures appears real. If the water host can no longer maintain
hollow structures that are large enough to accommodate guest molecules due to the
very high pressures, then the guest “solubility” in the solid water host would go down
and the clathrate hydrate would lose its source of thermodynamic stability.
Meanwhile, Yang et al. reported a new structure of xenon hydrate under atmospheric pressure at a cryogenic temperature [63]. The new phase was derived by
initial pressurization of sI xenon hydrate to 2.0 GPa at room temperature followed
by quenching to 77 K at atmospheric pressure. The new structure has a hexagonal
symmetry that is similar to MH-II and remains stable up to 160 K before decomposing
to the traditional sI phase [63].