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. 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 (C6Cl5NO2). 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