Calorimetric study of an induced nematic to smectic A phase transition in a binary system of two smectogens: Existence of double tricritical points

Abstract

In this work, we investigate a series of binary mixtures comprising two smectogenic compounds, 5DBT (containing terminal group, NCS and exhibiting smectic A1 phase) and 10OCB (having terminal group, CN and exhibiting smectic Ad phase). The binary system exhibits an induced N (nematic) phase within a definite concentration limit between X5DBT = 0.05 and X5DBT = 0.95. Utilizing high-resolution MDSC setup, we have carried out a detailed heat capacity measurement at the induced N to smectic A (SmA) transition. Analysis of the latent heat confirms the occurrence of two tricritical points: one having the McMillan ratio = 0.981 and the other corresponding to 0.983. A distinct pretransitional heat capacity change close to the transition temperature is observed. The renormalization group theory involving a correction-to-scaling factor, effectively describes this anomaly. The critical exponent α follows a distinct pattern when plotted against both mole fraction (X5DBT ) and McMillan ratio (TNA/TIN). As the system approaches the tricritical point (TCP), a consistent shift from 2nd order to 1st order N–SmA transition is noticed on each end of the phase diagram. Both the TCP shares a common McMillan ratio = 0.982. Additionally, the 3D–XY model is precisely attained near X5DBT = 0.687, corresponding to TNA/TIN = 0.909.