Orientationally disordered crystals: from the onset of slow dynamics to the glass transition

Calorimetric and X-ray experiments and molecular dynamics (MD) simulations have been performed on adamantane substitued plastic crystals. We show that orientationally disordered crystals provide a valuable alternative to the molecular liquid glass formers since they offer attractive possibilities to investigate the fundamental mechanism of the glass transition both in the dynamical decoupling regime (high temperature) and in the deep undercooled situation. This investigation confirms the existence of a second remarkable dynamical crossover at the temperature T-x found higher than the mode coupling theory critical T-c and consistent with previous NMR, Raman and MD studies. This allows us to confirm clearly the existence of a 'landscape-influenced' regime occurring in the temperature range [T-c-T-x] as recently proposed. Furthermore, a clear signature of cooperatively rearranging regions which develop on approaching T, is found. Instead of well defined independent domains as proposed by Adam and Gibbs, the cooperative regions can rather be identified here to short range order fluctuations whose spatial extent can be characterized by the associated correlation length. (C) 2002 Published by Elsevier Science B.V.