Nonlinear dielectric spectroscopy in a fragile plastic crystal

TL;DR

This study investigates the nonlinear dielectric behavior of a fragile plastic crystal, revealing heterogeneous dynamics, field-induced entropy changes, and cooperative molecular behavior, with implications for understanding glassy states.

Contribution

It provides the first detailed analysis of nonlinear dielectric properties in a fragile plastic crystal, highlighting unique mechanisms and spectral features associated with glassy dynamics.

Findings

Alteration of dielectric permittivity under high fields due to heterogeneous dynamics.

Detection of third harmonic component indicating cooperative molecular dynamics.

Strong temperature dependence of correlated molecules in fragile glass transition.

Abstract

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.