Experimental observation of multicritical points with TGB phases on a pure compound

High pressure studies for the n = 12 homologues of the 3-fluoro-4((R) or (S)-methylheptyloxy)-4'-(4-alkyloxy-3-fluorobenzoyloxy) tolanes series have been performed by thermobarometric analysis. The phase sequence presents two Twist Grain Boundary phases denoted TGB(A) and TGB(C). The pressure-temperature phase diagram has been determined; it exhibits a S*(C) phase, and an induced phase identified as S-A which does not exist under atmospheric pressure. The TGB(A) temperature stability domain versus pressure first increases, then rises to a maximum and finally disappears. Such phase behaviour leads to an experimental observation of two multicritical points S*(C)-S-A-TGB(A) and S-A-TGB(A)-N* for the pure compound under high pressures. This result confirms the existence of singular points previously predicted by the Renn-Lubensky theory.