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

Abstract

The freezing of the cooperative reorientational motions in orientationally disordered (OD) molecular crystals marks the so-called "glassy" transition, which may be considered a lower-dimensional version of the structural glass transition. While structural glasses display both positional and orientational disorder, in fact, in orientational glasses the disorder involves exclusively the orientational degrees of freedom of the constituent molecules, while the molecular centres of mass form an ordered lattice. We report here on a glass-forming system with even less degrees of freedom, namely the OD phase of a dipolar benzene derivative, pentachloronitrobenzene (C 6 Cl 5 N O 2 ). We probe the orientational dynamics of PCNB as a function of temperature and pressure * To whom correspondence should be addressed † Universitat Politècnica de Catalunya ‡ Università di Pisa 1 by means of dielectric spectroscopy at normal and high pressure and high-pressure density measurements, and show that the system exhibits a double primary relaxation feature associated with two distinct motions of the molecular dipole moment. After ruling out an interpretation in terms of primitive or intramolecular relaxations, we discuss an assignment of the double relaxation feature based on the material's anisotropy and on the comparison with discotic liquid crystals.

New Intermediate Polymorph of 1-Fluoro-adamantane and Its Second-Order-like Transition toward the Low Temperature Phase

“…It should be noticed here that the lowest temperature at which the structure was solved was 90K, close to the temperature (92K) at which reorientational motions are characterized by a relaxation time of the order of 100s 19 . Below such temperature, reorientational motions are frozen, according to the previous dynamic study 19 and thus, the LT phase becomes a low-dimensional glass [53][54][55] . It is obvious that such a LT phase cannot be the stable phase at 0K (the frozen disorder would confer a non-zero entropy) and the true stable phase remains at present elusive.…”

Polymorphism of 1,3-X-adamantanes (X = Br, OH, CH3) and the crystal plastic phase formation ability

“…Details of the experimental system can be found in ref. [42]. X-ray powder diffraction measurements.…”

Thermodynamic scaling of vibrational dynamics and relaxation

“…The above scaling is usually referred to as "temperature-density scaling" or "thermodynamic scaling" (TS). TS applies to van der Waals liquids, polymers, ionic liquids [7][8][9][10][11][12], liquid crystals [13] and plastic crystals [14] but not to all of the hydrogen-bonded liquids since the equilibrium structure of the liquid and its degree of hydrogen bonding are expected to change when temperature and pressure are changed [15]. Regarding network-bonded inorganic glass formers such as silica glasses, from the experimental and numerical studies it seems that the relation of Eq.1 keeps only locally, i.e.…”