Abstract
Usual ferroelectric compounds undergo a paraelectric-to-ferroelectric phase transition when the susceptibility of the electric polarization density changes its sign. The temperature is the only thermo-dynamic field that governs the phase transition. Chiral tilted smectics may also present an improper ferroelectricity when there is a tilt angle between the average long axis direction and the layer normal. The tilt angle is the order parameter of the phase transition which is governed by the temperature. Although the electric susceptibility remains positive, a polarization proportional to the tilt appears due to their linear coupling allowed by the chiral symmetry. Further complications come in when the chirality increases, as new phases are encountered with the same tilt inside the layers but a distribution of the azimuthal direction which is periodic with a unit cell of two (SmC∗ A), three (SmC∗ Fi1), four (SmC∗Fi2) or more (SmC∗ ) layers. In most of these phases, the layer normal is a symmetry axis so there is no macroscopic polarization except for the SmC∗Fi1 in which the average long axis is tilted so the phase is ferrielectric. By studying a particular compound with only a SmC∗ Fi2 and a SmC∗ phase, we show that we recover the uniformly tilted ferroelectric SmC∗ when applying an electric field. We are thus led to build field-temperature phase diagrams for this class of compounds by combining different experimental techniques described here.