Methane hydrate under high pressure

Abstract

The structural, electronic, and spectroscopic properties of a high-pressure phase of methane hydrate (MH-III) are studied by first principles electronic structure calculations. A detailed analysis of the atomic positions suggests that ionization of hydrogen-bonded water molecules occurs around 40GPa and centering or symmetrization of hydrogen-bonds occurs around 70 GPa. These pressures are much lower compared with ioninzation around 55 GPa and centering around 100 GPa in pure ice. The transition may be observed with low-temperature IR/Raman spectroscopy of OH stretching modes or neutron diffraction. PACS numbers: 62.50.+p,71.15.Pd Methane hydrate (MH), known as Burning Ice, is a special class of ice that contains methane molecules in cages or networks of hydrogen bonded water molecules. Low pressure phase of methane hydrate (MH-I) forms sI structure of cages [1]. MH-I, abundant in the deep ocean, has been attracting attention of the industry as a key material of new energy resource, whose amount is estimated twice as much as the total fossil fuel reserve [1].MH is also known as an important material for understanding the mystery of the atmosphere of Titan, the largest satellite of Saturn. The conventional theory [2] could not explain abundant methane gas in Titan's atmosphere because MH-I inside Titan was assumed to decompose into ice and methane around 1 or 2 GPa, and escape to the atmosphere to be photodecomposed in early stage of Titan's history. To understand this mystery is one of the goals of the Cassini-Huygens spacecraft, which started its journey in 1997 and will arrive at Saturn system in 2004 [3].On the earth, in 2001, Loveday et al. [4,5] discovered new phases of MH by X-ray and neutron diffraction experiments under high pressure: MH-I transforms to MH-II ( sH cage structure) at 1 GPa, and then to MH-III phase (orthorhombic filled ice structure) at 2 GPa, which survives at least up to 10 GPa. Other researchers reported similar high pressure phases [6,7]. Recently Hirai et al. [8] reported that MH-III survives up to 42 GPa at room temperature. Shimizu et al. [9] have measured the site-and pressure-dependence of CH-and OH-vibration frequencies in these phases up to 5.2 GPa. Discovery of these high pressure phases allows us a new explanation of abundant methane gas in Titan's atmosphere: methane gas may be reserved in thick layers of MH-III under Titan's surface and gradually emit to the atmosphere from the reservoir [4]. * Electronic address: tiitaka@riken.jp; URL: http://atlas.riken.go.jp/~iitakaIn this article, we focus on the features of MH-III as a promising material for investigateing centering or symmetrization of hydrogen bonds between water molecules. Advantage of using MH-III for studying the centering is that it is expected to occur at much lower pressure than that of pure ice (ice VII-ice X transition) [10,11,12,13,14,15,16,17,18,19,20], which may make the difficult high-pressure experiments easier. In the following, we calculate the crystal structure and vibrational spectra of MH-III by ...

High-pressure Raman study of methane hydrate "filled ice"

“…From the pressure dependence of the O-H stretching vibrational frequency of MH-III, the symmetrization pressure of hydrogen bond in MH-III phase could be estimated to be about 45 GPa, assuming that the symmetrization occurs when the O-H stretching vibration as a soft mode becomes zero frequency, where the squares of Raman frequency are linearly extrapolated to high pressure region [12,13] (see the inset of Figure 5). The expected symmetrization pressure of 45 GPa is consistent with the first-principles calculation [4]. …”

“…On the other hand, methane hydrate is considered as one of the dominant constituents of the outer planets and their moons, such as Neptune, Uranus and Titan. Therefore, it is important for planetary scientists to investigate the stability and physical properties of the high pressure phase of methane hydrate [3,4].…”

“…For preparing the sample of MH-III, we loaded fluid methane and distilled water into a sample chamber (0.1 mmφ in diameter, 0.1 mm in depth) of the DAC together with a small ruby ball for pressure calibration. After the loading into the DAC, we grew a single crystal of MH-II in a threephase equilibrium among MH-II, fluid CH 4 and water phases at about 1.0 GPa and 323 K. Figure 1(a) shows a synthesized single crystal of MH-II at 1.59 GPa at 296 K. Next, we compressed the single crystal of MH-II up to 1.91 GPa [see Figure 1(b)]. At this pressure, it took several days to complete the phase transformation to MH-III, as described below.…”

Effective potential for many-body interactions in some properties of the HFD-like solids

“…For cubic crystals, they are reduced to three components, C 11 ≡ C xxxx , C 12 ≡ C xxyy , and C 44 ≡ C yzyz (in the Voigt notation). These elastic coefficients can be determined by computing the stress generated by forcing a small strain to an optimized unit cell [53].…”

Support of Gulf of Mexico Hydrate Research Consortium: Activities to Support Establishment of a Sea Floor Monitoring Station Project

“…Currently, nuclear magnetic resonance (NMR) 13,14 , Raman [15][16][17][18][19][20][21] and Fourier transform infrared (FT-IR) spectroscopy [22][23][24] , along with X-ray diffraction 20,25 , gas chromatography 25 , and neutron diffraction 14 have provided a wealth of information on gas hydrates. In addition to fundamental studies, a great need exists for the capability to monitor in-situ hydrate formation and dissociation in both environmental and industrial settings.…”