Relaxation dynamics in plastic crystals

Abstract

We report dielectric data on six different plastic crystalline materials, namely 1-cyanoadamantane, adamantanone, pentachloronitrobenzene, cyclo-hexanol, ethanol, and meta-carborane, covering a frequency range of up to 14 decades and up to 20 GHz. Information on phase transitions, the α-relaxation, and relaxation processes beyond the α-relaxation are provided. The α-relaxation shows clear non-Debye behavior and varying degrees of deviation from thermally activated behavior. Our results reveal a generally rather low fragility of plastic crystals. In some of the investigated materials evidence for Johari–Goldstein type β-relaxations is obtained. In addition, the question of the so-called excess wing of the α-relaxation peak is addressed in detail. In all cases, it is either absent or can be ascribed to a β-relaxation submerged under the α-peak. Overall, the present work provides a broad database on the dielectric behavior of plastic crystals, and may be taken as a review of the dynamic phenomena occurring in these materials, many of them being observed also in structural glass formers.

Molecular diffusion and dc conductivity perfectly correlated with molecular rotational dynamics in a plastic crystalline electrolyte

“…Nonetheless, recent studies on compounds such as ethanol and glutaronitrile, which depending on the thermal treatment display (at the same temperature) either a plastic crystalline state or a supercooled liquid phase, have shown 27,36 that the characteristic diffusional frequency in the supercooled liquid is at all temperatures very close to that of reorientational motions in the plastic crystal. Our findings are therefore a further confirmation of the idea that the dynamics of glassforming systems is fundamentally determined by rotational motions; 27,[36][37][38][39][40] while the connection between translational and rotational timescales was until now only based on the observation of similar but not identical temperature dependence and frequency ranges in different phases, we report here a perfect correlation of translational and rotational degrees of freedom in the same phase. The correspondence of σ dc and f max originates because both quantities are fundamentally linked to the diffusion of molecular species between lattice defects, which can be thought to be limited by an effective "diffusional" viscosity η diff .…”

Double Primary Relaxation in a Highly Anisotropic Orientational Glass-Former with Low-Dimensional Disorder

“…Panels (a) and (b) of Figure 2 highlight the need of two dipolar processes (displayed as dotted lines) for modelling the dielectric spectra. It should be noticed that in previous work 19,40 only one HN function was employed to fit the loss curves, and this resulted in a slight discrepancy between the experimental data and the fit function in the high-frequency range of the spectra.…”

“…33 As far as orientationally disordered materials are concerned, an earlier work 18 based on the comparison of several OD phases concluded that the EW is absent and thus not a universal feature of such glass formers. However, few years later the same authors reported 19 the possible existence of an EW in PCNB. The phase behaviour of this molecule, which forms a layered structure of triangular planes, has been extensively studied.…”

“…[5][6][7] The reorientational motions may freeze upon cooling or pressurizing, resulting in an orientational glass: OD phases therefore exhibits a phenomenology that is analogous to that of structural glass formers. 4,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Due to the large number of degrees of freedom of its constituent molecules, molecular condensed-matter systems are characterized by a rich variety of dynamic processes and phases. In molecular materials forming structural or orientational glasses, the most important dynamics is the cooperative motion of the molecules, referred to as primary relaxation or α process, whose freezing marks the transition to the glass state.…”

“…The phase behaviour of this molecule, which forms a layered structure of triangular planes, has been extensively studied. 19,23,24,[34][35][36][37][38][39][40][41] A glass transition was observed by adiabatic calorimetry at approximately 185 K, which was assigned to the freezing of the orientational disorder within the hexagonal planes. 37 Similar results were found by Correia et al by DSC and thermally stimulated currents, with a reported transition temperature of 191 K. 38,39 Here, we show by means of broadband dielectric spectroscopy (BDS) that such phase is characterized by a double primary (collective) relaxation which is related to the low-dimensional character of the system and which results in a spectral landscape reminiscent of α relaxations characterized by an excess wing.…”