Thermodynamic and Kinetic Fragility of Freon 113: The Most Fragile Plastic Crystal

Abstract

We present a dynamic and thermodynamic study of the orientational glass former Freon 113 (1,1,2-trichloro-1,2,2-trifluoroethane, CCl 2 F-CClF 2 ) in order to analyze its kinetic and thermodynamic fragilities. Freon 113 displays internal molecular degrees of freedom that promote a complex energy landscape. Experimental specific heat and its microscopic origin, the vibrational density of states from inelastic neutron scattering, together with the orientational dynamics obtained by means of dielectric spectroscopy have revealed the highest fragility value, both thermodynamic and kinetic, found for this orientational glass former. The excess in both Debye-reduced specific heat and density of states (boson peak) evidences the existence of glassy low-energy excitations. We demonstrate that early proposed correlations between the boson peak and the Debye specific heat value are elusive as revealed by the clear counterexample of the studied case. DOI: 10.1103/PhysRevLett.118.105701 When a structurally disordered system is rapidly cooled to avoid crystallization, some properties, such as viscosity, show a dramatic increase down to the glass transition where the material reaches viscosity values comparable to those of a solid (10 12 Pa s), i.e., relaxation times of ≈100 s. Such behavior contrasts with that typical for most liquids at high temperatures, which usually exhibit a simple Arrhenius behavior of the relaxation time, τ ¼ τ 0 expðE a =k B TÞ, where the activation energy is temperature independent.Decreasing temperature relaxation time shows a stronger increase, faster than that of the Arrhenius law and accompanied with an increase of some characteristic cooperativity relaxation length. The viscosity (or τ) increase is generally characterized by recourse to the concept of the kinetic fragility [1,2], m ¼ fð∂ log τÞ=½∂ðT g =TÞg T¼T g , which accounts for the deviation of the Arrhenius temperature dependence.In terms of fragility index m, materials for which τ follow an Arrhenius law are known as "strong" glass formers, whereas "fragile" glass formers are those exhibiting super-Arrhenius behavior. For such cases, the temperature dependence of τ is given through the Vogel-FulcherTammann (VFT) expression,where the temperature T 0 is associated with an ideal glass transition and even with the so-called Kauzmann temperature [3], and the fragility strength parameter D is linked to the fragility parameter by. Typical strong glass formers (m ≈ 16, or D ≥ 100) are tetrahedral network liquids as SiO 2 or GeO 2 . The highest values of fragility for organic materials (exception made of polymers) have been found in cis-or trans-decahydronaphthalene (m ¼ 147 [4]). Another group of materials exhibiting glasslike properties is that of crystals with positional order and orientational disorder [5]. Such plastic phases are formed from the liquid and can be supercooled, giving rise to the so-called orientational glasses (OG) or "glassy crystals" [6][7][8][9]. They show typically low fragility, as cyclooctanol (m ¼ 33) [10,11]

Relaxation and vibrational properties in metal alloys and other disordered systems

“…The bump is significantly larger (by a factor 2) for Freon112 than for Freon113, which is consistent with the fact that Freon112 has more long-range [143], while data for Freon 113 were taken from Ref. [144].…”

Dynamic heterogeneity in an orientational glass

“…In these kind of systems, molecules may reorient by almost free rotations at the nodes of a crystal lattice and by cooling such a rotator-phase, a frozen orientationally disordered crystalline state (called glassy crystal or orientational glass) may be attained, exhibiting fully developed glassy behavior. Moreover, thermodynamic signatures similar to those common to vitrifying liquids are shown when approaching this state [9][10][11] .…”

“…Such model systems show trivial, noninteracting, equilibrium behaviors but nonetheless, interesting slow dynamics features appear due to restrictions on the allowed transition between configurations 8 . Disordered molecular crystals known as rotator-phases or plastic-crystals are simple systems (commonly with cubic lattice symmetry) exhibiting dynamical behaviors a) Electronic mail: nirvana.caballero@cab.cnea.gov.ar b) Electronic mail: zuriaga@famaf.unc.edu.ar c) Electronic mail: josep.lluis.tamarit@upc.edu d) Electronic mail: serra@famaf.unc.edu.ar which may at least partially be understood by recourse to results obtained from idealized models 9 . In these kind of systems, molecules may reorient by almost free rotations at the nodes of a crystal lattice and by cooling such a rotator-phase, a frozen orientationally disordered crystalline state (called glassy crystal or orientational glass) may be attained, exhibiting fully developed glassy behavior.…”

Critical Cooling Rate Prediction by the Reduced Glass Temperature and Fragility Index

“…(Top panel) Viscosity η as a function of T g / T . (Bottom panel) α ‐Relaxation time τ α as a function of T g / T …”