Review of high-resolution calorimetric studies of cyanoalkylbiphenyl and cyanoalkoxybiphenyl liquid crystals and related compounds.

The discovery of the cyanobiphenyl and related compounds in 1973 not only played a significant role in important innovations in displays and related technologies but also opened opportunities for testing of novel insights in understanding phase transitions and critical phenomena. In the study of these phenomena, high-resolution calorimetry made substantial contributions. In this paper, we present an overview of the high-resolution calorimetry results obtained during the past 50 years on the phase transitions that occur in the cyanobiphenyl compounds. For the isotropic–nematic and isotropic–smectic A transitions, their (weakly) first-order nature is well established and the temperature dependence of the pretransitional-specific heat capacity strongly points in the direction of a nearby tricritical point. For the nematic-smectic A transition, precise latent heat values of several binary liquid crystal (mainly of cyanobiphenyl type) mixtures give significant evidence on the Halperin, Lubensky and Ma prediction of the always weakly first-order nature of this transition. The dependence of the specific heat capacity critical exponent α on the width of the nematic range, resulting from coupling between the nematic and the smectic order parameters as pointed out by de Gennes, is well confirmed. [ABSTRACT FROM AUTHOR]