Critical behavior of dielectric permittivity in the vicinity of nematic–isotropic and smectic–nematic phase transitions in smectogenic binary mixtures

We report the phase diagram of a binary mixture composed of compounds, one having NCS terminal group (5DBT, showing smectic A1 phase) and the other with CN terminal group (10OCB, showing smectic Ad phase), exhibiting induced nematic phase in a certain concentration range (0.05 < x5DBT < 0.95). Measurement of static dielectric parameters on this binary system within the whole mesomorphic range is being reported. Pretransitional behavior at the nematic–isotropic and the smectic A–nematic phase transitions has been studied by measuring the static dielectric parameters in the entire mesomorphic range. Precise determination of discontinuity (ΔT) and the critical exponent (α) has been carried out for the nematic–isotropic phase transition. Effective molecular dipole moment and the angle of inclination with respect to the molecular long axis have been determined. High resolution density measurement is also carried out for one concentration x5DBT = 0.301 and from the molar volume so obtained, the discontinuity (ΔT) of the transition and the critical exponent are measured. Additionally, the nature of the N–SmA phase transition has been studied for a few mixtures from dielectric anisotropy measurements to explore the critical behavior of the N–SmA phase transition.