Tricritical behaviour of the smectic A to smectic C* transition for binary liquid crystal mixtures

The tricritical behaviour for the smectic A (S-A) to chiral smectic (S-C*) or smectic C (S-C) phase transition of four binary liquid crystal systems was investigated by C-13 NMR spectroscopy. The compounds 4'-(2-chloro-3-methylpentanoyloxy)-4-hexylo-1,1'-biphenyl (denoted C6) and 4'-(2-chloro-3-methylpentanoyloxy)-4-octyloxy-1,1'-biphenyl (C8) were used in their enantiomeric (L) and racemic (DL) forms. L-C6 and L-C8 exhibit a chiral smectic C (S-C*) to S-A phase transition, and DL-C6 and DL-C8 exhibit a S-C to S-A transition, which are first order. Information about the orientational ordering of the aromatic core of C6 and C8 was obtained from measurements of C-13 chemical shifts. C6 displays a significant jump at the phase transition, whereas the discontinuity of C8 is much weaker. The enantiomeric and racemic forms have similar order parameters and phase behaviour. Various mixtures of C6 and C8 with 4-butylolxyphenyl-4'-decyloxybenzoate (<(10)over bar>O (4) over bar), which has a second order S-C to S-A transition, show a crossover from a first order to a continuous second order transition via a tricritical point. The tricritical mole fraction X(TCP) of <(10)over bar>O (4) over bar in the mixtures was found to be 0 . 14+/-0 . 01 for DL-C6 and 0 . 13+/-0 . 01 for L-C6. For mixtures containing C8, the tricritical mole fractions were much smaller, and it was found that X(TCP)less than or equal to 0 . 02 for DL-C8 and X(TCP)less than or equal to 0 . 03 for L-C8.