X-ray and neutron scattering studies on vapor-deposited amorphous hydrogen hydrate

Clathrate hydrates are rare inclusion compounds capable of forming both crystalline and amorphous states, making them ideal materials for studying the effects of environmental factors such as symmetry and volume on the motions of guest molecules. In this study, amorphous hydrogen hydrate (a-HH) was formed at 7 K by using a low-temperature vapor deposition (VD) method and its structure and dynamics were investigated using synchrotron radiation x-ray diffraction (XRD), neutron diffraction, and quasielastic neutron scattering (QENS). The as-deposited a-HH possesses local cage-like space that accommodates hydrogen molecules, giving a structure clearly distinct from amorphous ice (a-ice). Upon heating, hydrogen molecules are released from a-HH at around 60 K, accompanied by a rapid development of the hydrogen-bond network. At 120 K, the structure approaches that of a-ice. The QENS spectrum observed during the VD displays localized motions of hydrogen molecules confined within a space with a characteristic radius of ∼2.6 Å. This space dimension is consistent with the free volume of small cages in hydrogen hydrates. The relaxation time of the motion of H2 molecules in the space is ∼50 ps, which is about one order of magnitude longer than the residence time reported for H2 confined in crystalline HH (c-HH). Thus, it was found that the behavior of H2 molecules in a-HH differs significantly from that in c-HH.