Polar order and tilt in achiral smectic phases.

Material with the phase sequence SmA-SmAP-SmCP is studied as an example of a system in which the spontaneous electric polarization and the molecular tilt develop independently at the SmA-SmAP and the SmAP-SmCP phase transition, respectively. The temperature dependence of the spontaneous electric polarization clearly shows a strong coupling between the polarization and tilt. The system exhibits also very strong precritical polarization and tilt fluctuations. Experimental observations are explained within the theoretical model.