Johari–Goldstein relaxation in orientationally disordered phase of hexa-substituted benzenes

• Dielectric spectroscopy and DSC are used to analyze solid solutions of PCP and HCB.
• Temperature dependence of relaxation rates follows Arrhenius equation.
• The main relaxation is a superposition of α- and βJG-process.
• The physical parameters of PCP–HCB solid solution follow Gordon–Taylor equation.
• Measurements in PCNB and PBT follow the same trend as PCP–HCB system.

We have studied the orientationally disordered phase in solid solutions of two hexa-substituted benzenes, pentachlorophenol (PCP) (C6Cl5OH) and hexachlorobenzene (HCB) (C6Cl6) at five concentrations encompassing the entire concentration range employing dielectric spectroscopy and differential scanning calorimetry. In the observed dielectric relaxation spectra, the primary or α-process is accompanied by an additional process on the high frequency side which gets resolved increasingly with increase in the concentration of non-polar HCB. The physical parameters of PCP–HCB solid solution are found to vary according to Gordon–Taylor equation. The resolved second process lies in close proximity to the β-process predicted using Coupling Model proposed by Ngai (1998, 2011). We obtained similar results in neat pentachloronitrobenzene (C6Cl5NO2) and pentabromotoluene (C6Br5CH3). In all the samples studied, the α-process is found to be more or less Arrhenius in its temperature dependence.