By means of NMR experiment and MD computer simulation we investigate the dynamical properties of a chloroadamantane orientationally disordered crystal. We find a plastic-plastic dynamical transition at T_x ~ 330 K in the pico-nanosecond regime. It is interpreted as the rotational analogue of the Goldstein crossing temperature between quasi-free 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. Close to T_x, we observe that two-step relaxations emerge. An interpretation which is based on the present study of a specific heat anomaly detected by a recent calorimetric experiment is proposed.
@article{arxiv.cond-mat/0004064,
title = {Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals},
author = {F. Affouard and E. Cochin and R. Decressain and M. Descamps},
journal= {arXiv preprint arXiv:cond-mat/0004064},
year = {2009}
}