XYbehavior for the heat capacity at nematic–smectic-A1liquid-crystal transitions

High-resolution ac calorimetric data are reported for three polar liquid-crystal systems: octyloxyphenylcyanobenzoxybenzoate (80PCBOB), a mixture of pentylphenylcyanobenzoyloxybenzoate (${\mathrm{DB}}_5$CN)+cyanobenzoyloxypentylstilbene (${\mathrm{C}}_5$ stilbene), and a mixture of ${\mathrm{DB}}_6$CN+${\mathrm{C}}_5$ stilbene. All three exhibit nematic–smectic-$A_1$ one liquid-crystal transition heat-capacity peaks well described by the XY critical exponent ${\alpha }_{\mathrm{XY}}$=-0.007. The critical amplitude ratio $A^{\mathrm{-}}$/$A^{+}$ (=0.99) agrees fairly well with the XY value and is inconsistent with the theoretically expected inverted-XY value. Correction terms also play an important role.