Heat capacity measurements near the nematic-smectic-A1 and smectic-A1-smectic-C transitions in octyloxyphenyl cyanobenzyloxy benzoate

The heat capacity Cp of octyloxyphenyl cyanobenzyloxy benzoate (8OPCBOB) has been measured with a high-resolution ac calorimetric technique near the nematic (N)-smectic-A1 (SmA1) and smectic-A1 (SmA1)-smectic- C (SmC) transitions. The SmA1-SmC transition is weakly first order with a Landau-like excess heat capacity (i.e., no excess Cp in the high-temperature SmA1 phase). A detailed analysis of the Cp variation associated with the second-order N-SmA1 transition is presented for 8OPCBOB and for previously published data on a mixture of pentylphenyl-cyanobenzyloxy benzoate (DB5CN) and cyanobenzoyloxypentyl stilbene (C5 stilbene). Both systems exhibit the same critical Cp behavior, which agrees well with the XY model. In particular, the critical amplitude ratio A-/A+ is 0.990, which agrees quite well with the theoretical ratio for the XY model but is qualitatively inconsistent with the expected inverted- XY value. 8OPCBOB and several other systems exhibit a pattern of systematic deviations just below Tc that may be intrinsic to all XY-like N-SmA transitions.