Phase diagram exhibiting a smectic-A–smectic-C–smectic-F meeting point

We report an observation of a smectic-A–smectic-C–smectic-F (A-C-F) meeting point in a binary liquid-crystalline system. Detailed high-precision x-ray measurements enabled us to characterize the meeting point as a triple point, a point at which the three phases coexist. A direct consequence of this result is the existence of a tricritical point (TCP) on the A-C boundary. Contrary to the known results, the TCP appears despite a large temperature range for the A phase and the occurrence of a nematic phase at a higher temperature. Bearing in mind that F is a tilted hexatic phase, it is suggested that a possible reason for this behavior is the molecular-tilt field arising from the coupling between the tilt and the bond-orientational-order parameters.