Relaxation dynamics and ionic conductivity in a fragile plastic crystal

We report a thorough characterization of the dielectric relaxation behavior and the ionic conductivity in the plastic-crystalline mixture of 60% succinonitrile and 40% glutaronitrile. The plastic phase can be easily supercooled and the relaxational behavior is investigated by broadband dielectric spectroscopy in the liquid, plastic crystalline, and glassy crystal phases. The very pronounced alpha-relaxation found in the spectra is characterized in detail. From the temperature dependence of the alpha-relaxation time, a fragility parameter of 62 was determined making this material one of the most fragile plastic-crystalline glass formers. A well-pronounced secondary and faint indications for a third relaxation process were found, the latter most likely being of Johari-Goldstein type. In addition, relatively strong conductivity contributions were detected in the spectra exhibiting the typical features of ionic charge transport.