Experimental and numerical signaturesof dynamical crossover in orientationallydisordered crystals

• Published in: Europhysics letters 2001-03, Vol.53 (5), p.611-617
• Main Authors: Affouard, F., Cochin, E., Decressain, R., Descamps, M.
• Format: Article
• Language: English
• Subjects: 61.43.-j; 64.70.Pf; 76.60.-k
• ISSN: 0295-5075; 1286-4854
• DOI: 10.1209/epl/i2001-00196-9

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} \simeq 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 Tx, 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 Tx which probably does not correspond to the $T_\ab{c}$ 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.