Pressure-induced amorphization of methane hydrate

Pressure-induced amorphization of methane hydrate has been investigated by molecular dynamics simulation. In accord with experimental results of Tulk et al., a crystalline -> amorphous transition was confirmed at 3.3 GPa where the water lattice collapsed around the encaged methane. Thermal annealing at 5.5 GPa allows the water to adopt a lower energy conformation with a denser structure. In both structures, methane molecules are immobilized and maintain long-range correlation as in the crystal. Consequently, both phases are predicted to revert back to the crystalline form upon decompression. Pressure-induced amorphization is a nonequilibrium process due to a mechanical instability; the interconversion between kinetically stable amorphous phases is a relaxation effect.