W e review dynami c processes i n supercooled liqui ds and glasses as studi ed by dielectri c spectroscopy. It i s the only experim ental techni que whi ch allows one to follow the tremendous slow-down of di OE usi ve motion of parti cles i n di sordered condensed matter over m ore than 18 decades in frequency or ti me. The di electri c techniques used are treated in detai l. As an i ntroducti on for non-speciali sts, the ti me and temperature evolution of the basi c spectral features associated with vari ous dynami c relaxation processes are di scussed in detai l. Among them are the structural relaxati on, the occurrence of fast processes and the boson peak. The relevance of these features for glass formation i s discussed. T he present arti cle may also serve as a review for recent experi mental and theoretical studi es on glassforming liqui ds.

Experimental evidence is provided that colossal dielectric constants Ју1000, sometimes reported to exist in a broad temperature range, can often be explained by Maxwell-Wagner-type contributions of depletion layers at the interface between sample and contacts or at grain boundaries. We demonstrate this on a variety of different materials. We speculate that the largest intrinsic dielectric constant observed so far in nonferroelectric materials is of order 10 2 .Materials exhibiting a colossal dielectric constant ͑CDC͒ ЈϾ10 3 have recently gained considerable attention. CDC behavior is of technical importance for applications using high-electronic materials, such as random access memories based on capacitive elements. Fundamental interest was initiated by the observation of CDC behavior in some high-T c parent compounds. 1,2 Indeed, CDC behavior may indicate a colossal polarizability, which was invoked in early polaronic and bipolaronic models as a possible mechanism for high-T c superconductivity. 3 During the last decade, similar observations of CDC behavior have been reported in an increasing number of materials, such as transition-metal oxides. 4 -6 Large dielectric constants are expected for ferroelectrics in a narrow temperature range close to T c or for systems with hopping charge carriers yielding a dielectric constant that diverges towards low frequencies. However, in various recent reports 1,2,4 -6 giant values of the dielectric constant were claimed to persist over broad temperature ranges and, when plotted as a function of frequency, revealing an almost constant low-frequency value and a steplike decrease of the dielectric constant towards higher frequencies. This steplike decrease, which is accompanied by a loss peak in the imaginary part of the permittivity, Љ, shifts exponentially to higher frequencies with increasing temperature, characteristic of Debye-like dipolar relaxation with a thermally activated relaxation rate. Several intrinsic physical interpretations have been given. Examples include almost incipient ferroelectricity in high-T c materials, 2 highly polarizable relaxation modes, 5 or a relaxorlike slowing down of dipolar fluctuations in nano-sized domains. 6 However, in Ref. 7 it was suspected that extrinsic effects may play a role in the CDC reported in Ref. 6.In the present paper we provide evidence that many of these observations are not intrinsic in origin and we speculate that most, if not all, of the CDC's reported so far are based on Maxwell-Wagner-type extrinsic effects. 8 We will promote the notion that the most natural explanation of apparent CDC's is contact effects and that in ceramic samples grain boundary effects may play a similar role and further ''enhance'' the dielectric constant. At these interfaces ͑metal-to-insulator contacts, intergrain boundaries͒ depletion layers are formed yielding Maxwell-Wagner-type relaxations when measured by standard dielectric techniques that use metallic electrodes and two-point contact configurations. Thus, while some of the reports may ...

Many transition-metal oxides show very large ("colossal") magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energystorage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today's most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2−xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant. a

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.

Dielectric loss spectra of glass-forming propylene carbonate and glycerol at temperatures above and below T(g) are presented. By performing aging experiments lasting up to five weeks, equilibrium spectra below T(g) have been obtained. During aging, the excess wing, showing up as a second power law at high frequencies, develops into a shoulder. The results strongly suggest that the excess wing, observed in a variety of glass formers, is the high-frequency flank of a beta relaxation.