The phase transitions from a smectic-$C$ to a tilted hexatic phase are known to form a first-order transition line that terminates in an isolated critical point, beyond which there is supercritical evolution of bond-orientational order without a thermodynamic transition. Recent experiments strongly suggest that smectic-$A$ ($\mathrm{Sm}-A$) to hexatic-$B$ ($\mathrm{Hex}-B$) transitions are also first order, albeit in some cases very weakly so. A generalized smectic-hexatic phase diagram is proposed that unifies phase transitions involving tilted- and untilted-smectic and hexatic phases. Current information about $\mathrm{Sm}-A–\mathrm{Hex}-B$ phase transitions is summarized and discussed in the context of quasicritical behavior.

• Received 4 August 1997

DOI:https://doi.org/10.1103/PhysRevE.57.3015