Dielectric spectroscopy of de Vries-type smectic-A* -smectic-C* transitions.
We report results of dielectric investigations on a number of ferroelectric liquid crystalline (FLC) compounds with different degrees of layer shrinkage in the smectic-A(*) -smectic-C(*) phase transitions. With a decreasing extent of layer shrinkage the investigated FLCs exhibit a significantly increasing soft-mode absorption, decreasing leading Landau coefficient alpha, and a considerably broader mean-field regime. We explain these tendencies by the fact that the low layer shrinkage materials come closer to the diffuse cone model of de Vries than to the common model of rigid rods which maintain their orientational order during tilting. In the case of the diffuse cone model the tilt-angle fluctuations are decoupled from variations of the smectic layer spacing, which explains the observed behavior.