  In this work we provide a thorough examination of the nonlinear dielectric
properties of a succinonitrile-glutaronitrile mixture, representing one of the
rare example of a plastic crystal with fragile glassy dynamics. The detected
alteration of the complex dielectric permittivity under high fields can be
explained considering the heterogeneous nature of glassy dynamics and a
field-induced variation of entropy. While the first mechanism was also found in
structural glass formers, the latter effect seems to be typical for plastic
crystals. Morover, the third harmonic component of the dielectric
susceptibility is reported, revealing a spectral shape as predicted for
cooperative molecular dynamics. In accord with the fragile nature of the glass
transition in this plastic crystal, we deduce a relatively strong temperature
dependence of the number of correlated molecules, in contrast to the much
weaker temperature dependence in plastic-crystalline cyclo-octanol, whose glass
transition is of strong nature.
