Orientational relaxations in solid (1,1,2,2)tetrachloroethane

Abstract

We employ dielectric spectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassy transition at 156±1 K, corresponds to a cooperative motion by which each molecule rotates by 180º around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectric spectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids. 2 INTRODUCTIONWhile conventional (atomic) solids are made of atomic constituents with only translational degrees of freedom, so that their structure is totally determined by translation symmetry and fundamental excitations are vibrational in character, in molecular solids the constituent molecules possess also orientational (as well as internal) degrees of freedom, which lead to a richer variety of possible solid phases and to the existence of rotational excitations such as librations and orientational relaxations. A molecular solid can display complete translational and rotational order, as in a molecular crystal, or complete rototranslational disorder, as in a molecular glass. In between these two extremes, molecular solids also display phases (known as Orientationally disordered (OD) phases are generally formed by relatively small globular molecules such as derivatives of methane, 1,2,3 neopentane, 4 adamantane 5 or fullerene, 6 or by small linear ones such as ethane derivatives 7,8,9 and dinitriles. 10 OD solids exhibit many of the phenomenological features of glass formers, displaying in particular a cooperative rotational motion, called α relaxation, that undergoes a continuous, dramatic slow-down upon cooling, 11,12 leading in some cases to a glass-like 3 transition associated with rotational freezing. 13,14 Contrary to structural glasses, which do not exhibit any long-range order, OD phases are characterized by a translationally ordered structure and can therefore be more thoroughly characterized with the help of methods that exploit the translational symmetry such as Bragg diffraction, lattice models, or solid-state NMR spectroscopy. Even more importantly, since as mentioned OD phases are generally formed by molecular species with a simple structure ...

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“…6,[13][14][15][16][17][18][19] Among them, the Johari-Goldstein β relaxation process exhibited even by rigid molecules is quite common and can be interpreted on the basis of the energylandscape picture as jumps between the basins 20,21 within a metabasin and generally follows the predictions of the Coupling Model [22][23][24] The existence of dynamic processes has been made evident even for highly ordered systems such as translationally ordered phases with an intrinsic statistical disorder involving only the site occupancy of one or few atoms of the (rigid or flexible) molecular entities. [9][10][11][12][25][26][27][28][29] In these cases, the distinct site occupancy probabilities (called fractional occupancies) reflect the existence of perfectly defined, discrete allowed angular orientations of the molecules which undergo reorientational jumps between the allowed orientations, in contrast to the undefined and hardly quantifiable dynamics of translationally and orientationally disordered phases such as the liquid state. The study of these minimally disordered crystalline systems with few and quantifiable disordered configurations can allow the precise identification of the different dynamic processes observed experimentally, as well as on the required minimal disorder for the emergence of the so-called universal thermal anomalies of the glass state.…”