Filled ice structure of gas hydrates-a density functional study

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 40 GPa and centring or symmetrization of hydrogen bonds occurs around 70 GPa. These pressures are much lower compared with ionization around 55 GPa and centring around 100 GPa in pure ice. The transition may be observed with low-temperature IR/Raman spectroscopy of OH stretching modes or neutron diffraction