Analytical Sciences vol:17 Special Iss. SI pages:S81-S84
Using a novel photopyroelectric ac calorimetric technique we have investigated the phase transition lines between the nematic and the smectic A phase in two binary mixture systems of liquid crystalline compounds. In the system composed of heptyl-(7AB) and octylazoxybenzene (8AB) the transition changes from first-order to second-order at a tricritical point and the critical exponent cc for the specific heat capacity anomaly indicates cross-over from tricritical to XY critical behavior along the second-order part of the transition line. The system of pentylcyanobiphenyl (5CB) and 4-n-propylcyclohexyl-carboxylate (D55) shows injected smectic A behavior and due to the strong composition dependence of the transition temperature the observed critical and tricritical specific heat capacity behavior is fully Fisher renormalized.