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

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. Although structural glasses display both positional and orientational disorder, in orientational glasses, however, the disorder involves exclusively the orientational degrees of freedom of the constituent molecules, while the molecular centers of mass form an ordered lattice. We report here on a glass-forming system with even fewer degrees of freedom, namely, the OD phase of a dipolar benzene derivative, pentachloronitrobenzene (C6Cl5NO2). We probe the orientational dynamics of pentachloronitrobenzene as a function of temperature and pressure by means of dielectric spectroscopy (and high-pressure density measurements), and we show that, due to its anisotropy, the system exhibits a double primary relaxation feature associated with two distinct motions of the molecular dipole moment. This complex relaxation scenario shows a scaled dependence on the thermodynamic variables (P,T), with all relaxation times collapsing onto a single curve for each relaxation when plotted versus a specific-volume-dependent scaled variable TV γ. Our findings are in line with the recent prediction by Dyre and co-workers of the existence of a hidden-scale invariance also in van der Waals crystalline materials.