High-resolution ac specific-heat and x-ray investigations of the smectic-F—smectic-Htransition of HBHA

High-resolution ac specific-heat measurements of the smectic-$F$—smectic-$H$ transition of $\mathrm{N}-(4-n-\mathrm{heptyloxybenzylidene})-4-n-\mathrm{hexylaniline}$ (HBHA) are reported. A hysteresis of 160 mK in the transition temperature is measured for the first time for transformations in thermotropic liquid crystals. The specific heat, obtained during heating, is larger and changes more rapidly at the transition than during the cooling cycle. This behavior of the specific heat may be explained by assuming that dislocations and defects play an important role at this transition. High-resolution x-ray measurements of the smectic layer thickness were made with an accuracy of better than 250 ppm. The temperature dependence of the smectic-$F$ layer thickness of HBHA is qualitatively different from that of TB5A and its homologs.