Methyl-group dynamics from tunneling to hopping in NaCH3CO2·3 H2O: Comparison between a crystal and its glassy counterpart

Neutron-scattering measurements are carried out in order to study the temperature evolution of methyl-group dynamics in the same sample of sodium acetate trihydrate in crystalline and glassy state. The results in the crystalline sample are analyzed according to the usual single-particle assumption, and those in the glass in terms of a Gaussian distribution of single-particle potentials, this distribution resulting from the structural disorder present in the glass. It is found that the average potential barrier for the glass takes, within the experimental error, the same value as the single barrier in the crystal. The standard deviation of the distribution takes a value similar to those obtained in the quite different structural glasses (polymers) that were studied up to now. The reliability of some approximations introduced by the model for the dynamics of the individual methyl groups in the glass are tested in the crystalline phase