Experimental observation of multicritical behaviors in pressure-temperature diagrams of pure compounds with twist grain boundary phases.

In order to study critical behaviors of pure compounds anticipated by Renn and Lubensky's theoretical calculations, high-pressure experiments have been performed on six tolan series homologous to the 3-fluoro-4-[(R) or (S)-methylheptyloxy]-4'-(4"-alkoxy-2",3"-difluorobenzoyloxy), which exhibit the twist grain boundary (TGB(A) and TGB(C)) mesophases. Measurements were carried out by thermobarometric analysis. Six pressure-temperature phase diagrams are determined. The studies establish the existence of multicritical points twisted-smectic-C (S*(C))-TGB(C)-TGB(A), TGB(C)-TGB(A)-cholesteric nematic (N*) and S*(C)-TGB(C)-N* for single component systems in pressure-temperature (P-T) phase diagrams. Our results also show that a pressure increase has the same effect on the (P-T) phase diagrams as a decreasing number of carbon atoms in the aliphatic chain; then pressure change can have the same effects on intermolecular interactions as those observed when shortening the molecular length. The existence of transition lines with negative slope values was also found. The experimental results are in qualitative agreement with the model proposed by Renn and Lubensky.