Nematic-Smectic-ATricritical Point in Alkylcyanobiphenyl Liquid Crystals

From adiabatic-scanning calorimetric results it is demonstrated that in mixtures of 9CB and 10CB, two compounds of the alkylcyanobiphenyl ($n\mathrm{CB}$) series, the nematic ($N$) to smectic-$A$ ($A$) transition becomes first order for narrow $N$ ranges. From the latent heats a tricritical point is located in the phase diagram. Measurements for 7CB+ 8CB mixtures show that the specific-heat exponent $\alpha$ depends on the width of the $N$ range for continuous $\mathrm{AN}$ transitions.