Optical rotatory power in tilted smectic phases

Optical Rotatory Power measurements have been done on different chiral tilt ed smectics which lead to reproducible results. The high-temperature uniaxi al SmA and SmCalpha* phases do not show any ORP due to the lack of in-plane anisotropy at the mesoscopic level (i.e, a few layers). At lower temperatu re one gets the synclinic C* and the anticlinic C-A* phases with a helical precession around the layer normal which is right-handed for C* and left-ha nded for C-A*. These phases are separated in a finite temperature range by a set of two so-called "ferrielectrics" SmCFi1* and SmCFi2*, which are not necessarily new phases, between which the handedness changes sign at a give n temperature, while the change from synclinic to anticlinic tilt angle org anization is spread out over the "ferrrelectric" temperature range. In orde r to reconcile all experimental observations, otherwise apparently incompat ible, it is proposed that the "ferrielectric" region is a diphasic one wher e the in-plane organization changes smoothly from the synclinic type to the anticlinic one.