Is There an Excess Wing in the Dielectric Loss of Plastic Crystals?

Abstract

The plastic crystals ortho-and meta-carborane and 1-cyanoadamantane are investigated by dielectric spectroscopy for frequencies 10 23 # n # 10 9 Hz, paying special attention to the spectral shape of the loss peaks. For the carboranes, we provide clear experimental evidence that the high-frequency wing follows a single power law over many decades in frequency. The excess wing, which shows up in a variety of glass-forming liquids as a second power law at high frequencies, is completely absent in the carboranes. In 1-cyanoadamantane, a weak excess wing shows up which can be ascribed to a Johari-Goldstein b process. We conclude that the excess wing is a property of supercooled liquids only. [S0031-9007(99)

α and β relaxation dynamics of a fragile plastic crystal

“…But a mesophase can exist between the completely ordered crystalline phase and the translationally and orientationally disordered liquid phase, the so called plastic phase or orientationally disordered (OD) phase [2,3]. In the plastic phase, the centers of mass of the molecules have spatial long range order, forming a lattice which generally has high symmetry (such as cubic, quasi-cubic or rhombohedral [2, 3, 4]), but there is only short-range order with respect to the orientational degrees of freedom [5].As for glass-forming liquids, the typical phenomenology of a glass transition also can be realized in various plastic crystals when the temperature is decreased, but in this case only the orientational degrees of freedom are frozen, yielding the formation of a "glassy crystal"[6], also called OD glass.Concerning the dynamics of OD phases, dielectric spectroscopy has been revealed to be a useful tool to understand the complex dynamics of these phases [7,8,9,10,11,12,13].Based on these works there seem to be some general dynamic features of OD phases and its glasses: They are rather strong (following the definition of Angell [14]) and they follow the Böhmer relation between non-exponentiality of the α relaxation and fragility [15,16].On the contrary, the so called Nagel scaling [17] does not seem to work for these phases [7,11,18]. In addition there is only a weak or no β-relaxation at all and the excess wing, showing up as a second power law at the high frequency flank of the α peak in many canonical glass formers [17,19,20,21], is either absent in plastic crystals [11] or can be ascribed to a weak secondary relaxation [7,9].…”

Emergence of glassy-like dynamics in an orientationally ordered phase

“…16 In some cases, the question of the appearance of the so-called excess wing of the α-relaxation for this kind of material is still a matter of controversy, 16 and it is not even excluded that excess wing and β-relaxation are expressions of the same microscopic mechanism. Nevertheless, as far as we know, there are few experiments reporting the existence of relaxation processes in OG from orientationally ordered phases and only one showing two relaxation processes:…”

“…An alternative homogeneous explanation attributes the secondary relaxation phenomena to small-angle reorientations of all the molecules. 14 Although the JG relaxation is believed to be present in all types of glass formers, and thus sometimes thought as a precursor of the α-relaxation, for many "strong" glasses (according to the "strong/fragile" Angell classification 15 ) it has not been observed either as a clear peak 6 or as a shoulder 16 on the imaginary part of the dielectric susceptibility. The possibility that the characteristic relaxation time for the JG relaxation is not much shorter than that of the α-relaxation cannot be ruled out.…”

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

“…[25][26][27] The spectral landscape of secondary relaxations of structural glass formers is further enriched by the existence of the so-called excess wing (EW), that is, of an excess dielectric loss on the highfrequency side of the α-relaxation process. 17,18,28,29 Based on comparative studies of several structural glass formers, some authors 30 have proposed that the EW could actually be a nonresolved secondary relaxation process hidden below the high-frequency wing of the primary α-relaxation. In recent work on structural glasses, application of high pressure confirmed that the EW is indeed a "submerged" secondary relaxation.…”

“…[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.…”