Relaxation dynamics in plastic crystals

We report dielectric data on six different plastic crystalline materials, namely 1-cyanoadamantane, adamantanone, pentachloronitrobenzene, cyclo-hexanol, ethanol, and meta-carborane, covering a frequency range of up to 14 decades and up to 20 GHz. Information on phase transitions, the alpha-relaxation, and relaxation processes beyond the alpha-relaxation are provided. The alpha-relaxation shows clear non-Debye behavior and varying degrees of deviation from thermally activated behavior. Our results reveal a generally rather low fragility of plastic crystals. In some of the investigated materials evidence for Johari-Goldstein type beta-relaxations is obtained. In addition, the question of the so-called excess wing of the alpha-relaxation peak is addressed in detail. In all cases, it is either absent or can be ascribed to a beta-relaxation submerged under the alpha-peak. Overall, the present work provides a broad database on the dielectric behavior of plastic crystals, and may be taken as a review of the dynamic phenomena occurring in these materials, many of them being observed also in structural glass formers. (C) 2002 American Institute of Physics.