DOI: 10.1209/epl/i2001-00196-9


Europhys. Lett., 53 (5) , pp. 611-617 (2001)

Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals

F. Affouard, E. Cochin, R. Decressain and M. Descamps

Laboratoire de Dynamique et Structure des Matériaux Moléculaires, CNRS ESA 8024, Université Lille I - 59655 Villeneuve d'Ascq Cedex, France

(Received 25 October 2000; accepted 22 December 2000)

Abstract
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 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. We show also that two-step relaxations emerge close to T_x which probably does not correspond to the 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.

PACS
64.70.Pf - Glass transitions.
76.60.-k - Nuclear magnetic resonance and relaxation.
61.43.-j - Disordered solids.


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