Structural and dynamic studies on vapor-deposited amorphous methane hydrate

Abstract

Methane hydrate has been studied extensively not only as a future energy resource but also due to interest in quantum and classical rotations of methane molecules. In this study, amorphous methane hydrate (a-MH) was formed at 7 K by a low temperature vapor deposition method. To investigate the local amorphous structure and the dynamics of guest methane molecules, x-ray and neutron diffraction, quasi-elastic neutron scattering, and adiabatic calorimetry experiments were performed for both as-deposited and annealed samples. After the deposition, a-MH has a disordered and distorted cage-like structure. Annealing at 120 K promotes local hydrogen-bond ordering while keeping the amorphous state, and the resulting, more well-defined cages align as in structure I hydrate. Crystallization (decomposition to methane gas and cubic ice) occurred at 165 K. In the as-deposited sample, the motions of the guest methane molecules are more hindered, and the rotational potential barrier is also higher. While after annealing, the motions are more like spherical rotation, as in structure I hydrate.