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At the phase transition from SmC* or SmC* A to the tilted hexatic phase the spontaneous polarization, tilt angle and the length of the helical pitch significantly decrease. Dielectric spectroscopy exhibits a single relaxation process in the vicinity of these transitions, which can be attributed to fluctuations of bond orientational order (BOO) coupled to the tilt and polarization. The anomalies of both the relaxation frequency and the dielectric strength of the mode at the transition as well as behavior of spontaneous quantities are discussed on the basis of a new theory, which takes into account a competition of the space modulation of tilt and BOO order parameters.

Optical measurement and microscopic observations have been performed of single crystals of (TMA)2ZnCl4 with the aim to establish its phase diagram in the coordinates temperature-uniaxial stress σ. Stress-induced shifts of transition temperatures have been evaluated. For stresses exceeding 4.5 MPa two new monoclinic phases are reported, tentatively labeled as VIII and IX, both with the monoclinic axis b.

Data are presented on dielectric properties of Rb2CoCl4 crystals. A wide tail of enhanced permittivity extends into the ferroelctric phase. It shows a relaxation in the MHz region with a wide distribution of relaxation times. On lowering temperature, the decrease in permittivity is accompanied by dielectric softening as revealed by decreasing mean relaxation frequency.

While the ground state in the ferroelectric lock-in phases of A2BX4 compounds is single domain, a “dielectric tail” of enhanced permittivity extending into these phases is known to exist, connected with the presence of domain walls. We review available data including recent measurements on purified Rb2ZnCl4 and existing models. To interpret the results, temperature and field induced changes of both wall density and restoring force coefficient must be taken into account.