A New Kind of A-A Transition: Studies on Binary Mixtures of Terminally Substituted Cyano and Nitro Compounds

High pressure, Xray and miscibility studies on binary mixtures of terminally substituted cyano and nitro compounds have led to the observation of two kinds of A-A transitions, viz., Ad-Ad and Ad-A1, the former being observed for the first time. The signatures of these transitions, which are not observable either optically or by differential scanning calorimetry, are seen dramatically in the smectic A -nematic phase boundary in the P-T plane. Xray results show that the Ad-Ad transition is from a partially bilayer smectic A with a temperature-independent layer spacing to one with a temperature-dependent layer spacing.