Calorimetric study of the incommensurate smectic-A phase in DB₇OCN + 8OCB

Abstract

Mixtures of heptyloxyphenylcyanobenzoyloxybenzoate (DB₇OCN) and octyloxycyanobiphenyl (8OCB) exhibit partial bilayer smectic-A (S_Ad) and bilayer smectic-A (S_A2) phases when the mole fraction of 8OCB is less than 0.24. For mixtures containing larger amounts of 8OCB, an incommensurate smectic-A (S_Aic) phase is observed between S_Ad and S_A2. The heat capacity variation C_p(T), determined with an ac calorimetric technique, is qualitatively different for samples going directly from S_Ad to S_A2 and those exhibiting the S_Ad–S_Aic–S_A2 sequence. The direct S_Ad–S_A2 results indicate a continuous supercritical evolution of the S_A2 phase. The results in the S_Aic range support the presence of a new phase, as indicated by X-ray studies, rather than a broad region of coexisting bulk S_Ad and S_A2 phases.