The dynamics in the orientationally disordered phase is often cooperative in nature and is designated as primary (or a-) relaxation, the kinetic freezing of which leads to “orientational glass” or “glassy crystal”, analogous to the liquid glass formers where the kinetic freezing of the translational as well as the rotational degrees of freedom leads to “structural glasses”. Besides the primary relaxations, often discussed in this context is the Johari-Goldstein (JG) secondary relaxation arising due to entire molecule motion which is believed to be universal. Despite JG-relaxation are often assumed to play essential role in the glass transition, their microscopic origin is still controversially discussed.
In this talk, I will discuss the dynamics in orientationally disordered phase of the hexa-substituted benzenes (HSBs), studied employing dielectric spectroscopy and differential scanning calorimetry. The orientationally disordered phase of HSBs occurs in less symmetrical monoclinic, triclinic, and rhombohedral structures, and the deviation of the temperature dependence of the structural relaxation rates from the Arrhenius law is less pronounced in these materials as compared to orientationally disordered systems occurring in cubic phase as well as liquid glass forming systems. Further, I will discuss how the positional order changes the microscopic structural features in orientational glasses as compared to structural glasses via our study of secondary relaxations in these systems.