Under pressure studies of TGBA and TGBC phases on pure compounds

High pressure experiments have been performed on three homologous (n=9 to 11) of the 3-fluoro-4-[(R) or (S)-methylheptyloxy]-4′-(4″-alkoxy-2″,3″-difluorobenzoyloxy) tolans series which exhibit the TGB A and TGB c phases. The transitions were detected by thermobarometric analysis. For n=9, the TGB 4 temperature stability domain versus pressure first increases, then rises to a maximum and finally disappears in the S A -TGB A TGB A -N * multicritical point. An induced S A phase appears under pressure for n = 10 leading to the observation of S C *-S A -TGB A multicritical point. For the first time, a (P-T) phase diagram exhibiting simultaneously and clearly the both domains TGB A and TGB C phases is obtained for n=11. These results confirm the existence of singular points previously predicted by the Renn and Lubensky theory.