OPTICAL-PROPERTIES AND PHENOMENOLOGICAL THEORY OF FERRI-FERRI PHASE-TRANSITIONS IN ANTIFERROELECTRIC LIQUID-CRYSTALS

Optical rotatory power and Raman scattering experiments on chiral liqu id crystals belonging to a series which exhibits one or two ferrielect ric phases have been performed. The results of these non-perturbative techniques show that, depending upon the aliphatic chain length, a fir st order transition with a jump in sign and value of the helical pitch , or a divergence and sign reversal of the pitch with no transition ca n occur in these systems. These observations are in agreement with a g eneralized Landau-de Gennes phenomenological model predicting that the ferrielectric phases, when both exist, should be isostructural.