Nematic-smectic-A-smectic-Cpolycritical point: Experimental evidence and a Landau theory

Differential-scanning-calorimetry measurements and optical-microscopy observations are reported on mixtures of two stable liquid crystalline compounds, one of which has the nematic, smectic-A and smectic-C phases, while the other has only nematic and smectic C. The phase diagram for this system is found to have the following features: (i) a line of second-order nematic-smectic-A transitions with large pretransitional effects, (ii) a line of continuous smectic-A---smectic-C transitions, (iii) a line of first-order nematic-smectic-C transitions with very weak pretransitional effects, and (iv) a multicritical point where these three lines meet. A phenomenological Landau-like theory is presented, which qualitatively describes the observed phase diagram. On the basis of this work it is argued that the nematic-smectic-C transition is always of first order.