Specific heat study of first order smectic A to smectic B (Hexatic) transitions in n-alkyl-4'-n-pentanoyloxy biphenyl-4-carboxylates

A recently synthesized and characterized homologous series of liquid crystal compounds (n4C00BC) is shown by high resolution a.c. microcalorimetry experiments (on the n=3, 6 homologues) to most likely exhibit Smectic A to Smectic B (Hexatic) (SmBH) phase transitions exclusively. This is in partial contrast to earlier tentative assignments, based on low resolution calorimetry, X-ray and microscopy studies, which had crystal smectic B (SmBX) for n=4-8 and SmBH for n=3 and 9. The observed SmA-SmBH thermal anomalies exhibit strong fluctuation contributions but are clearly first order. Experimental results are compared with the extensively characterized n-alkyl-4'-alkoxy-biphenyl-4-carboxylates (nmobe's) and discussed in the context of Goodby's empirical molecular structure rules for the existence of the SmBH, and in the context of a generic phase diagram (SmA,BX,BH) recently predicted by Aharony et al. The latter illustrates the physical content of our earlier suggestion that short range hexagonal positional order is responsible for the ubiquitous tricriticality of the SmA-SmBH phase transition.