doi:10.1016/j.cplett.2004.02.064 
Copyright © 2004 Elsevier B.V. All rights reserved.
Single-crystal growth of the high-pressure phase II of methane hydrate and its Raman scattering study
Tatsuya Kumazaki a, Yoshihiro Kito a, Shigeo Sasaki a, b, Tetsuji Kume b and Hiroyasu Shimizu 
, 
, a, b
a Environmental and Renewable Energy Systems, Graduate School of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
b Department of Materials Science and Technology, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
Received 8 January 2004;
Revised 22 January 2004.
Available online 16 March 2004.
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Abstract
A single crystalline methane hydrate (MH) of high-pressure phase II was synthesized in a diamond anvil cell at P≈1.0 GPa and 50 °C through its decomposition line. Its shape is a hexagonal pillar that suggests MH-II is the hexagonal structure. In situ Raman spectra of the C–H stretching vibrations in the guest CH4 molecules present a broad band at 0.94 GPa and separate into two Raman bands at P≈1.3 GPa and room temperature. By considering the Raman intensities and the pressure dependences of their frequencies, we suggest that there exist two or three CH4 molecules in a large cage if five small cages are fully occupied in the MH-II (sH) phase.
Fig. 1. Pressure–temperature phase diagram of H2O and CH4–H2O system indicated by solid and broken lines, respectively. Broken lines A and B are the decomposition curves of MH-sI and MH-II, respectively [18]. Dotted lines C and D at room temperature are from MH-sI to MH-II and from MH-II to MH-III phase-transition points at P≈0.9 and 1.9 GPa, respectively [5 and 7]. The bold arrow indicates the process of single-crystal growth of MH-II at P≈1.0 GPa and 50 °C.
Fig. 2. Photomicrographs of high-pressure methane hydrate phase II (MH-II) in the sample chamber (diameter of 0.5 mm, depth of 0.3 mm) of DAC: (a) Coexisting state of a small MH-II seed crystal, fluid CH4, and water at P≈1.0 GPa and 50 °C; (b) a single crystal at P≈0.94 GPa and room temperature in MH-II phase, showing the hexagonal pillar surrounded by water; (c) a single crystal of MH-II surrounded by water at P=1.30 GPa. Brown spots and patches appeared in the crystal; (d) MH-II single crystal surrounded by water at P=1.36 GPa. Many brown spots and patches grew gradually; (e) MH-II is surrounded by ice-VI at P=1.63 GPa; (f) with increasing pressure, MH-III appeared with surrounding ice-VI at P=1.83 GPa.
Fig. 3. Pressure dependence of typical Raman spectra for C–H stretching modes of encaged CH4 molecules in MH-sI, MH-II, and MH-III phases. Separated Raman bands in MH-II were systematically deconvoluted into two bands (dotted lines). These bands at lower and higher frequencies correspond to CH4 molecules encaged in five small cages and one large cage, respectively (see text).
Fig. 4. Pressure dependence of Raman frequencies (wavenumbers) for C–H stretching modes of encaged CH4 molecules in MH-sI, MH-II, and MH-III phases. In MH-II, open circles indicate frequencies with the first increase in pressure from 0.9 to 1.36 GPa. At P≈1.4 GPa the surrounding water froze into ice-VI, and then the pressure in the DAC decreased to 1.11 GPa due to the decrease in volume of H2O. Solid circles thus indicate frequencies with the second increase from 1.11 to 1.71 GPa.
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50 cm−1, being based on the clear distinction between the guest and host vibrations by deuteration of the host cage.
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