Calorimetric study of the smectic-A—smectic-Cphase transition in liquid crystals

Abstract

High-resolution ac calorimetric measurements have been made on $\mathrm{azoxy}-4,4^{\prime }-\mathrm{di}-\mathrm{undecyl}-\alpha -\mathrm{methylcinnamate}$ (AMC-11), butyloxybenzylidene heptylaniline (4O.7), heptyloxybenzylidene hexylaniline (7O.6), and heptyloxybenzylidene butylaniline (7O.4). The heat capacity $C_P$ exhibits a sharp, asymmetric peak near the second-order smectic-$A$—smectic-$C$ transition. The $C_P$ values increase in the smectic-$C$ phase as the transition temperature $T_c$ is approached and then decrease abruptly at $T_c$. These data are well described by a mean-field Landau model and are not consistent with the heliumlike critical behavior expected from the de Gennes model. It is also shown that it is very difficult to distinguish between meanfield and $\mathrm{XY}$ critical behavior on the basis of data obtained only in the smectic-$C$ phase.