Dynamic heterogeneity in the glass-like monoclinic phases of CBrnCl4−n, n = 0,1,2

Abstract

Glassy dynamics of rigid molecules is still a matter of controversy: the physics behind the relaxation process at time scales faster than that ruled by the viscosity, the so called Johari-Goldstein process, is not known. In this work we unravel the mechanism of such a process by using a simple molecular model in which the centers of mass of the molecules are forming an ordered lattice, and molecular reorientation is performed by jumps between equilibrium orientations. We have studied the dynamics of simple quasi-tetrahedral molecules CBr(n)Cl(4-n), n = 0, 1, 2, in their monoclinic phases by means of dielectric spectroscopy and nuclear quadrupole resonance: the first technique allows to measure in a broad time scale but it is insensitive to molecular particularities, while the second has a restricted time window but senses the movement of each chlorine atom separately. The dynamic picture emerging from these techniques is that the secondary relaxation process is related to the different molecular surroundings around each nonequivalent atom of the molecule. Dynamical heterogeneities thus seem to be the cause of the secondary relaxation in this simple model of glass.

Dynamic heterogeneity in an orientational glass

“…The analysis of the isostructural CCl 4 shows that nonequivalent molecules in the unit cell perform reorientational jumps at different time scales due to their different crystalline environments. These results support the conclusion that the dynamic heterogeneity is intimately related to the secondary relaxation observed in these compounds 27,28,31 .…”

“…The dynamics of the monoclinic phases of CBr n Cl 4−n , n=0,1,2 compounds has been studied by means of dielectric spectroscopy and nuclear quadrupole resonance (NQR) spectroscopy 27,28 in the temperature range 100 -250 K and 80 -210 K, respectively. The former technique allows the measurement of the dynamic response within a broad time scale but it is insensitive to fine details of molecular motions, whereas the latter has a restricted time window but monitors the movement of individual chlorine atoms.…”

“…On the other hand, the NQR experiments are limited to a temperature range between 77 K and 140 K, with the upper end determined by the broadening of the signal 28 . The picture that emerges from the combined analysis is that the three compounds have a very similar dynamic evolution in the monoclinic phase as a function of temperature 27,28 . The analysis of the isostructural CCl 4 shows that nonequivalent molecules in the unit cell perform reorientational jumps at different time scales due to their different crystalline environments.…”

Dynamic characterization of crystalline and glass phases of deuterated 1,1,2,2 tetrachloroethane

“…In some of these cases, it has been demonstrated that the main glass features show up, pointing out the relevance of the orientational degrees of freedom as far as the glass properties are considered. [20][21][22] Also, it should be stressed that for all those disordered systems the collective dynamics of the entities cannot be decoupled from the molecular or localized excitations, and thus the use of pertinent techniques, which encompass the different origins of disorder, should provide important highlights to the glass transition problem.…”

Stable, metastable and unstable phases of halogen methane derivatives

“…The presentation will describe the stable and metastable phases of those compounds revealed by the uncommon inclusion of the pressure variable as well as by the analyses of the two-component systems sharing them. Moreover, the emergence of unstable phases (glasses) [2] within the low-temperature domain will be analyzed and the similarities with the canonical glassformers will be discussed. …”