Phase diagram and dielectric relaxation studies of n-octyl-isothiocyanato-biphenyl (8BT) in the crystalline E phase under high pressure

The pressure-temperature phase diagram of n-octyl-isothiocyanato-biphenyl ( 8BT) in the pressure range up to 250 MPa (2.5 kbar) and the temperature ran ge 250-400 K was established with the aid of DTA. At 1 atm the substance ex hibits exclusively CrE polymorphism. At pressures above 190 MPa, the cleari ng line splits showing an additional phase which is not yet identified. Die lectric relaxation measurements on the CrE phase of 8BT were performed in t he pressure range 0.1-120 MPa and the temperature range 304-345 K. A Debye- type relaxation process was observed in the frequency range 100 Hz-1 MHz. T he longitudinal relaxation time tau, characterizing the molecular reorienta tions around the short axis, was analysed with respect to the pressure and temperature, yielding the activation volume, Delta V-#= RT(partial derivati ve 1n tau /p)(T), and activation enthalpy, Delta (#) H = R(partial derivati ve 1n tau/partial derivativeT(-1))(p), respectively. The results are compar ed with analogous data obtained recently for similar compounds having other liquid crystalline phases (N, SmA).