Nonlinear dielectric spectroscopy in a fragile plastic crystal

Abstract

In this work we provide a thorough examination of the nonlinear dielectric properties of a succinonitrileglutaronitrile 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 plasticcrystalline cyclo-octanol, whose glass transition is of strong nature.

Investigation of nonlinear effects in glassy matter using dielectric methods

“…Fig. 1(c)) [9,10,24,25,26,30,31]. As shown by Richert and co-workers [9,24,77], within the framework of the so-called box model [26,99] a selective transfer of field energy into the heterogeneous regions can be assumed, leading to nonlinear effects in both the 1 susceptibility [9,24] and the higher harmonics [77].…”

“…The latter can exhibit glassy freezing, which in linear dielectric spectroscopy leads to very similar phenomenology as in canonical glass formers [102]. Only recently, nonlinear dielectric spectroscopy was also reported for plastic crystals [21,31,118,119]. Figure 11 shows the nonlinear 1 response of two plastic-crystalline materials, cyclo-octanol [21] and a mixture of 60% succinonitrile and 40% glutaronitrile (60SN-40GN) [31].…”

“…Only recently, nonlinear dielectric spectroscopy was also reported for plastic crystals [21,31,118,119]. Figure 11 shows the nonlinear 1 response of two plastic-crystalline materials, cyclo-octanol [21] and a mixture of 60% succinonitrile and 40% glutaronitrile (60SN-40GN) [31]. Again, the logarithmic difference of the high-and low-field results is plotted.…”

“…2(b)). Consequently, as mentioned in section 1, various nonlinear dielectric measurements were interpreted in terms of molecular cooperativity (e.g., [12,13,18,20,21,23,31]). However, aside of the saturation and chemical effects, several other mechanisms were also considered to give rise to dielectric nonlinearity.…”

“…In the main part, we will summarize experimental results obtained by nonlinear dielectric spectroscopy. We will especially concentrate on the results from our group in the past years [16,18,21,23,30,31], which partly are the outcome of a very fruitful collaboration with the groups of F. Ladieu, G. Biroli, and J.-P. Bouchaud.…”