The slowing down of molecular dynamics when approaching the glass transition generally proceeds much stronger than expected for thermally activated motions. This strange phenomenon can be formally ...ascribed to a temperature-dependent activation energy E(T). In the present work, via measurements of the third-order nonlinear dielectric susceptibility, we deduce the increase of the number of correlated molecules N(corr) when approaching the glass transition and find a surprisingly simple correlation of E(T) and N(corr)(T). This provides strong evidence that the noncanonical temperature development of glassy dynamics is caused by a temperature-dependent energy barrier arising from the cooperative motion of ever larger numbers of molecules at low temperatures.
Glasses are ubiquitous in daily life and technology. However, the microscopic mechanisms generating this state of matter remain subject to debate: Glasses are considered either as merely hyperviscous ...liquids or as resulting from a genuine thermodynamic phase transition toward a rigid state. We show that third- and fifth-order susceptibilities provide a definite answer to this long-standing controversy. Performing the corresponding high-precision nonlinear dielectric experiments for supercooled glycerol and propylene carbonate, we find strong support for theories based on thermodynamic amorphous order. Moreover, when lowering temperature, we find that the growing transient domains are compact—that is, their fractal dimension df = 3. The glass transition may thus represent a class of critical phenomena different from canonical second-order phase transitions for which df < 3.
Ionic liquids (ILs) are salts that are liquid close to room temperature. Their possible applications are numerous, e.g., as solvents for green chemistry, in various electrochemical devices, and even ...for such "exotic" purposes as spinning-liquid mirrors for lunar telescopes. Here we concentrate on their use for new advancements in energy-storage and -conversion devices: Batteries, supercapacitors or fuel cells using ILs as electrolytes could be important building blocks for the sustainable energy supply of tomorrow. Interestingly, ILs show glassy freezing and the universal, but until now only poorly understood dynamic properties of glassy matter, dominate many of their physical properties. We show that the conductivity of ILs, an essential figure of merit for any electrochemical application, depends in a systematic way not only on their glass temperature but also on the so-called fragility, characterizing the non-canonical super-Arrhenius temperature dependence of their ionic mobility.
Deep eutectic solvents with admixed lithium salts are considered as electrolytes in electrochemical devices, such as batteries or supercapacitors. Compared to eutectic mixtures of hydrogen-bond ...donors and lithium salts, their raw-material costs are significantly lower. Not much is known about glassy freezing and rotational–translation coupling of such systems. Here, we investigate these phenomena by applying dielectric spectroscopy to the widely studied deep eutectic solvent glyceline, to which 1 and 5 mol % LiCl were added. Our study covers a wide temperature range, including a deeply supercooled state. The temperature dependences of the detected dipolar reorientation dynamics and ionic direct current (dc) conductivity reveal the signatures of glassy freezing. In comparison to pure glyceline, the lithium admixture leads to a reduction of ionic conductivity, which is accompanied by a reduction of the rotational dipolar mobility. However, this reduction is much smaller than that for deep eutectic solvents (DESs), where one main component is lithium salt, which we trace back to the lower glass-transition temperatures of lithium-doped DESs. In contrast to pure glyceline, the ionic and dipolar dynamics become increasingly decoupled at low temperatures and obey a fractional Debye–Stokes–Einstein relation, as previously found in other glass-forming liquids. The obtained results demonstrate the relevance of decoupling effects and glass transition to the enhancement of the technically relevant ionic conductivity in such lithium-doped DESs.
This topical review provides an overview of the dielectric properties of a variety of organic charge-transfer salts, based on both, data reported in literature and our own experimental results. ...Moreover, we discuss in detail the different processes that can contribute to the dielectric response of these materials. We concentrate on the family of the 1D (TMTTF)2X systems and the 2D BEDT-TTF-based charge-transfer salts, which in recent years have attracted considerable interest due to their often intriguing dielectric properties. We will mainly focus on the occurrence of electronic ferroelectricity in these systems, which also includes examples of multiferroicity.
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