Binary systems of smectics A with enhanced nematic phase

Phase diagrams were determined for binary mixtures consisting of a monomolecular smectic A (d ~ l) and of a bimolecular one (d > 1). Three types of phase diagrams are observed : i) No smectic phase is observed in the concentration range of 0.2-0.4 to 0.6-0.8 mole fraction. ii) Only significant lowering of the phase stability is observed in the range specified in i). iii) No lowering of the smectic phase stability is observed or, if it occurs, it is very small. The observed nematic gap between the mono- and bimolecular SA phases is related to the incomensurate periodicities of the smectic layers. It is suggested that this phenomenon has a mechanism similar to that of the « re-entrant » nematic phase formation.