Heat capacity near the nematic–smectic-Atransition of a weakly polar, very wide nematic range liquid crystal

We report high-resolution ac-mirocalorimetry and birefringence measurements near the nematic--smectic-A (N--Sm-A) transition in a weakly polar very wide nematic range (T_N--Sm-A/T_N-I=0.768) liquid-crystal compound 4-n-pentyl-cyclohexyl-4-n-butyl-cyclohexyl benzoate (Merck S-1223). The data show a very weak anomaly at the transition with {α}=αeff{~}0.2. The very wide nematic range notwithstanding, birefringence data show coupling between the nematic and sematic-A order parameters, which can explain αeff>0. However, the presence of an underlying discontinuity in Cₚ at Tc suggests that αeff>0 may alternatively be due to evolution from classical mean-field behavior with Gaussian fluctuation corrections to asymptotic critical behavior, a scenario that is shown to be consistent with the Ginzburg criterion.