Volumetric study of liquid-crystal heptyloxyazoxybenzene

The differential dilatometer technique is used in a volumetric study of the phase transitions in $p$-heptyloxyazoxybenzene. The pretransition volume changes associated with liquid-crystal order-parameter fluctuations are interpreted in terms of the Landau-de Gennes theory. This theory is consistent with the behavior in the isotropic-nematic transition region. For the nematic to smectic-$C$ pretransition the behavior is anomalous: As the smectic-$C$ transition point is approached the expansion in the nematic phase remains constant, rather than increasing as in the typical pretransition behavior.