Critical heat capacity at the smectic A ± smectic C transition in a partially ̄ uorinated liquid crystal

A high resolution a.c. calorimetric study of a phenyl pyrimidine derivative with one alkyl and one per ̄ uorinated end group shows that the excess heat capacity associated with the smectic A± smectic C transition exhibits critical ̄ uctuation behaviour that is consistent with the three-dimensional XY universality class. This is in contrast to the usual mean-® eld SmA± SmC behaviour and the near tricritical ̄ uctuation behaviour reported recently in several other compounds. It is speculated that smaller than usual correlation lengths are the cause for the wide ̄ uctuation region in this compound.