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For the first time the low frequency relaxation process in two smectic phases (smectic A and smectic C) was studied at elevated pressures with the aid of DTA and dielectric spectroscopy. The substance studied, 2-(4-hexyloxyphenyl)-5-octyl-pyrimidine (6OPB8 in short) exhibits the nematic (N) - S A  - S C  phase sequence. The p-T phase diagram was established with DTA. However, the S A  - S C  transition was not observed in the DTA, but could be detected by dielectric relaxation measurements. The dielectric relaxation time measured as function of temperature and pressure, τ ∥ (p,T), enabled us to calculate the activation volume, Δ # V =RT(∂lnτ /∂p) T , and activation enthalpy, Δ # H = R(∂lnτ /∂T −1 ) p . It was found that Δ # V(SA) &gt; Δ # V(SC) and Δ # H(N) ≫ Δ # H(SA) &gt; Δ # H(SC), indicating that the molecular rotations around the short axes are more feasible in the tilted SC than in the orthogonal S A  phase.

DTA and Dielectric Studies of a Substance with the Nematic, Smectic A, and Smectic C Polymorphism at Ambient and Elevated Pressures