Calorimetric study of the smectic-Ahexatic-B phase transition of a liquid-crystal binary mixture

The phase diagram of the liquid crystal n-propyl-4-n-decyloxy-biphenyl-4- carboxylate [3(10)OBC] and 4-propionyl-4-n-heptanoyloxy-azobenzene (PHOAB) binary mixture has been investigated by heat-capacity studies and optical observations. The heat-capacity data reveal a continuous change in the character of the smectic-Ahexatic-B phase transition which is continuous in 3(10)OBC and becomes first order in PHOAB. It is proposed that this crossover effect is due to the coupling of the spatial fluctuations of the smectic layers to the hexatic parameter.