α and β relaxation dynamics of a fragile plastic crystal

Abstract

We present a thorough dielectric investigation of the relaxation dynamics of plastic crystalline Freon112, which exhibits freezing of the orientational degrees of freedom into a glassy crystal below 90 K. Among other plastic crystals, Freon112 stands out by being relatively fragile within Angell's classification scheme and by showing an unusually strong β-relaxation. Comparing the results to those on Freon112a, having only a single molecular conformation, points to the importance of the presence of two molecular conformations in Freon112 for the explanation of its unusual properties. 1 INTRODUCTIONGlass phenomena occur when a dynamically disordered system (e. g. aliquid, plastic crystal or paramagnet) freezes as a function of external temperature or pressure devoid of long range order. In this freezing process, one or more degrees of freedom of atoms or molecules continuously slow down, reaching the so called glass transition when their dynamics has a characteristic time, generally chosen to be 10 2 s (for recent reviews on glass transition see, e.g., [1]). Since in the liquid phase there is translational and orientational disorder, the glass transition of canonical glass formers is associated with the freezing of these two degrees of freedom. 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].

Nonlinear dielectric spectroscopy in a fragile plastic crystal

“…Most plastic crystals (including cyclo-octanol 22 ) can be characterized as rather "strong" glass formers 17,23 within the strong/fragile classification scheme used to account for the different degrees of deviations from Arrhenius behavior of the -relaxation time. 24 Thus 60SN-40GN is an especially Electronic mail: peter.lunkenheimer@physik.uni-augsburg.de interesting system as it represents one of the rare examples 25 of a relatively fragile plastic crystal. 20,23,26 Thus it is well suited to check if the relation between apparent energy barrier and N corr , suggested for structural glass formers, 6 holds in plastic crystals, too: For a fragile material this relation should lead to a significantly stronger variation of N corr than the relatively weak temperature dependence observed in cyclo-octanol.…”

Interplay between intramolecular and intermolecular structures of 1,1,2,2-tetrachloro-1,2-difluoroethane

“…26 Nevertheless, studies of the structure at the inter-and intramolecular scale showed that the interplay of features at these two different length scales may explain the complex dynamics of F-112. [27][28][29] The present work is focused on the determination of the intramolecular structure of F-112, its interplay with intermolecular ordering in the liquid phase, and a deep analysis of the short-range order, and is organized as follows: After a short description of the experimental and simulation procedures, a fit method to obtain molecular parameters from the neutron diffraction data is presented. The intramolecular structure of F-112 will then be obtained from experimental data.…”

Thermal properties of halogen-ethane glassy crystals: Effects of orientational disorder and the role of internal molecular degrees of freedom

“…[14][15][16][17][18][19] Halogen-ethane derivatives have been thoroughly characterized macroscopically (heat capacity, thermodynamic magnitudes, phase transitions, etc.) and microscopically (molecular dynamics, crystallographic structure, relaxation dynamics, molecular structure, etc.)…”

“…seeking to find a relation between its physical properties and its intermolecular interactions, as well as its intramolecular features, which can influence both the short-and the long-range order. [14][15][16][17][18][19][20][21][22] F-112 and F-113 are archetypal examples in which a mixture of conformers is present, and for which a complicated sequence of calorimetric transitions appears. Figure 1 shows the two F-112 conformers, gauche and trans, and the two F-113 conformers, C 1 and C s .…”