We have studied the hexatic-$B$–smectic-$A$ $\left(\mathrm{Hex}B\text{−}\mathrm{Sm}A\right)$ transition in $n$-hexyl-$4^{\prime }\text{−}n$-pentyloxybiphenyl-4-carboxylate (65OBC) by means of a high-resolution ac photopyroelectric (PPE) calorimetric technique. A procedure for the interpretation of the PPE data, which allows the detection of an internal heating source due to strain annealing and/or latent heat, has been applied. We have found that the strain present in the sample depends on the kinetics of formation of the smectic phase once the sample is cooled from the isotropic one. The strain field keeps memory of this kinetics and can be only partially annealed on decreasing the temperature or cycling it around the $\mathrm{Hex}\text{−}\mathrm{Sm}A$ transition. A reversible ordering-disordering process has been found at $T_c$ and has been explained in terms of the competition between the order parameter variation with temperature and the constraints imposed by the disorder. The results confirm that the transition has a weakly first-order character with a specific heat critical exponent that disagrees with the available theoretical predictions. Our data show the importance of the disorder in 65OBC and we tried to clarify what would be the consequence of this result in theoretical modeling devoted to solving the puzzle of the $\mathrm{Hex}B\text{−}\mathrm{Sm}A$ transition in this compound.

• Received 29 July 2005

DOI:https://doi.org/10.1103/PhysRevE.74.041707