Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals

By means of NMR experiment and MD computer simulation we investigate the dynamical properties of a chloroadamantane orientationally disordered crystal. We nd a plastic-plastic dynamical transition at T-x similar or equal to 330 K in the pico-nanosecond regime. It is interpreted as the rotational analogue of the Goldstein crossing temperature between quasifree diffusion and activated regime predicted in liquids. Below T-x, NMR experimental data are well described by a Frenkel model corresponding to a strongly anisotropic motion. At higher temperatures, a drastic deviation is observed toward quasi-isotropic rotational diffusion. We show also that two-step relaxations emerge close to T-x which probably does not correspond to the T-c of the Mode Coupling Theory. An interpretation which is based on the present study of a specific heat anomaly detected by a recent calorimetric experiment is proposed.