On the nature of the smectic A-to-nematic phase transition of 8CB

Density as a function of temperature was measured in the smectic A and nematic phases in the immediate vicinity of the smectic A-to-nematic transition temperature (T_AN′) of p-n-octyl-p′-cyanobiphenyl (8CB). The very small volume change obtained at the smectic A-to-nematic (A-N′) phase transition of "pure" 8CB confirms that it is weakly first-order. There is also evidence that the A-N′ phase transition of pure 8CB takes place in two steps; the first is a very weak first-order transition, involving an extremely small volume change, to a phase of "intermediate" order which extends over about 0.07 K and then undergoes a second transition, which may or may not be second-order, to the nematic phase.