Critical behaviour near the nematic-smectic-C and nematic-smectic-A phase transitions of a binary system: a comparative study from calorimetric, optical and dielectric measurements.

Detailed heat capacity, birefringence and dielectric anisotropy measurements have been carried out near the nematic – smectic-C (NC) and the nematic – smectic-A (NA) phase transitions of a mesogenic binary system. Important feature is the existence of two tricritical points one for the NC transition and other for the NA transition. The dependence of critical exponent (α) is well explained in terms of temperature ratios (TNC/TNI and TNA/TNI) for the NC and NA phase transitions. Both transitions exhibit non-universal behaviour and showing effective critical exponents in between tricritical and 3D-XY values. However, the nematic range necessary for the NC tricritical point is quite large (>20°C). The enthalpy, phase shift and imaginary heat capacity are also analogous to the nature of the critical exponent (α). Moreover, within a very small concentration range, the smectic-A – smectic-C (AC) transition line appears. Although the smectic-A width above the AC transition is quite small~2K, heat capacity and birefringence data clearly confirm second-order nature. Additionally, there exists a region where the NC, NA and AC line segments converge, i.e. a NAC multicritical point. At this point, the latent heat for the NC, NA and AC transition lines disappears, indicating an obvious second-order nature of the NAC multicritical point. [ABSTRACT FROM AUTHOR]