Dynamical behaviour of methane in a clathrasil (dodecasil 3C) studied by quasi-elastic neutron scattering

The dynamics of CH4 molecules enclosed in the two types of cavity of different volume in dodecasil 3C have been studied by means of incoherent neutron scattering at temperatures 2 K⩽T⩽500 K. While the motion of the guest molecules in the large cavities gives rise to significant quasi-elastic scattering, it was impossible to resolve the movements within the smaller cavities. At T>180 K, two quasi-elastic contributions were observed, belonging to the translational and rotational components of a free diffusive motion of a CH4 molecule within a restricted volume. Indications of a phase transition at T=250 K were found. The analysis of the incoherent structure factor shows that the molecules become more localized with decreasing temperature. A broad distribution of excitation energies at T=2 K indicates frozen-in disorder.