Cooperativity and Heterogeneity in Plastic Crystals Studied by Nonlinear Dielectric Spectroscopy

Abstract

The glassy dynamics of plastic-crystalline cyclo-octanol and ortho-carborane, where only the molecular reorientational degrees of freedom freeze without long-range order, is investigated by nonlinear dielectric spectroscopy. Marked differences to canonical glass formers show up: While molecular cooperativity governs the glassy freezing, it leads to a much weaker slowing down of molecular dynamics than in supercooled liquids. Moreover, the observed nonlinear effects cannot be explained with the same heterogeneity scenario recently applied to canonical glass formers. This supports ideas that molecular relaxation in plastic crystals may be intrinsically nonexponential. Finally, no nonlinear effects were detected for the secondary processes in cyclo-octanol.

Nonlinear dielectric spectroscopy in a fragile plastic crystal

“…1,2,3, 4,5,6,7,8). It was found that the response of glassy matter to high electrical fields goes far beyond the rather trivial polarization saturation effects investigated in some pioneering works on nonlinear dielectric properties (e.g., Refs.…”

“…However, in this plastic crystal the detected cooperativity seems to lead to much weaker slowing down of molecular dynamics than in the canonical glass formers. 8 Unfortunately, until now cyclo-octanol seems to be the only plastic crystal were higher harmonics of the susceptibility were reported and further measurements are needed to check for the possible universality of these findings.…”

“…Interestingly, while some of these effects resemble those found in structural glass formers (i.e., supercooled liquids), in several respects the nonlinear dielectric response of plastic crystals seems to show peculiar differences, despite their linear response behaves very similar to that of conventional glass formers. 17 An important example is the variation of the main relaxational response (the  relaxation) as obtained for high ac fields: In two independent studies, 8,15 the typical relaxation peaks revealed in the frequency dependence of the dielectric loss, ε"(), were found to exhibit a broadening on both flanks when measured at high fields of several 100 kV/cm. In contrast, in supercooled liquids such fieldinduced broadening was only found at the high-frequency flank of the loss peaks, 1,5 which could be well explained assuming a distribution of relaxation times caused by dynamical heterogeneities.…”

“…13,14 Very recently, nonlinear dielectric spectroscopy was also applied to the so-called plastic crystals. 8,15,16 While the molecules in these materials have long-range translational order, their reorientational degrees of freedom exhibit considerable disorder and glassy freezing at low temperatures. Pronounced nontrivial nonlinear behavior was also found in this class of glasslike materials.…”

“…1, 18 Based on measurements of the third-order harmonic component of the susceptibility  3 , in Ref. 8 indications for molecular cooperativity governing the glassy freezing of plastic-crystalline cyclooctanol were found. Moreover, in agreement with the behavior of various supercooled liquids, 6 the cooperativity was demonstrated to scale with the apparent temperaturedependent energy barrier (note that according to a recent work 19 anharmonicity may play an addition role).…”