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

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Abstract
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.