  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.
