EXPERIMENTAL-OBSERVATION OF MULTICRITICAL POINTS WITH TGB PHASES ON APURE COMPOUND

High pressure studies for the n = 12 homologues of the 3-fluoro-4((R) or ethylheptyloxy)-4'-(4-alkyloxy-3-fluorobenzoyloxy) tolanes series h ave been performed by thermobarometric analysis. The phase sequence pr esents 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 exi st 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 existenc e of singular points previously predicted by the Renn-Lubensky theory.