Volume 53, Number 5, March 2001
|Page(s)||611 - 617|
|Section||Condensed matter: structure, mechanical and thermal properties|
|Published online||01 December 2003|
Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals
Laboratoire de Dynamique et Structure des Matériaux Moléculaires,
CNRS ESA 8024,
Université Lille I -
59655 Villeneuve d'Ascq Cedex, France
Accepted: 22 December 2000
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 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 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
© EDP Sciences, 2001
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