Calorimetric investigations of liquid-crystal compounds exhibiting almost no layer-shrinkage behavior through the smectic-A-smectic- C* transition.

Heat-capacity measurements have been made on liquid-crystal compounds exhibiting almost no layer-shrinkage (NLS) behavior through the $\mathrm{Sm}\text{\penalty1000-\hskip0pt}A--\mathrm{Sm}\text{\penalty1000-\hskip0pt}{C}^{*}$ phase transition. The transition was found to be second order for two of the substances studied. It was found that the heat-capacity anomaly accompanying a second-order $\mathrm{Sm}\text{\penalty1000-\hskip0pt}A--\mathrm{Sm}\text{\penalty1000-\hskip0pt}{C}^{*}$ transition with NLS behavior is quite similar to that observed for typical antiferroelectric liquid crystals of the 4-(1-methylheptyloxycarbonyl)phenyl ${4}^{\ensuremath{'}}$-octyloxybiphenyl-4-carboxylate (MHPOBC) group, showing three-dimensional (3D) $XY$ behavior in the vicinity of the transition. On the other hand, for one compound which shows a weakly first-order transition, the anomaly is almost symmetric above and below ${T}_{c}$, with a significant fluctuation effect in the $\mathrm{Sm}\text{\penalty1000-\hskip0pt}A$ phase. For this compound, the critical behavior of the heat-capacity anomaly is almost tricritical in the immediate vicinity of ${T}_{c}$, while away from ${T}_{c}$ the behavior can be explained with the 3D $XY$ model. This suggests that the underlying transition with the 3D $XY$ critical behavior is driven to almost being tricritical but remaining weakly first order. No indication of low-dimensional character in the critical behavior was found in both cases.