Results of broadband dielectric investigations of thick layered CuInP₂S₆ and CuInP₂Se₆ crystals in direction parallel and perpendicular to the layers are reported. No dielectric anomaly indicates spontaneous polarization appearance in the direction parallel to the layers in CuInP₂S₆ and CuInP₂Se₆ crystals. In this direction the dielectric dispersion is mostly caused by Maxwell-Wagner relaxation and ionic conductivity. The typical for ferroelectric phase transition dielectric anomaly is observed only in the direction perpendicular to the layers in both crystals. The activation energy of DC conductivity is much higher in the direction perpendicular to the layers, which confirms ionic nature of electrical conductivity in both crystals.

The valence fluctuations which are related to the charge disproportionation of phosphorous ions P4++P4+→P3++P+5 are the origin of ferroelectric and quantum paraelectric states in Sn(Pb)2P2S6 semiconductors. They involve recharging of SnPS3 or PbPS3 structural groups which can be represented as half-filled sites in the crystal lattice. Temperature–pressure phase diagram for Sn2P2S6 compound and temperature-composition phase diagram for (PbySn1–y)2P2S6 mixed crystals, which include tricritical points and where a temperature of phase transitions decrease to 0 K, together with the data about some softening of low energy optic phonons and rise of dielectric susceptibility at cooling in quantum paraelectric state of Pb2P2S6 are analyzed by GGA electron and phonon calculations and compared with electronic correlations models. The anharmonic quantum oscillators model is developed for description of phase diagrams and temperature dependence of dielectric susceptibility.

In bulk proper uniaxial ferroelectrics, double antiferroelectriclike hysteresis loops are observed in the case of ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystal. The quantum-anharmonic-oscillator model is proposed for the description of such a polarization-switching process. This phenomenon is related to the three-well local potential of the spontaneous-polarization fluctuations at a distinctive negative ratio of coupling constants that correspond to intersite interaction in the given sublattice and interaction between two sublattices of the modeled ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystal structure. The data obtained can be used for the development of multilevel-cell-type memory technology.

Mixed Cu(In${}_x$Cr${}_{1-x}$)P${}_2$S${}_6$ crystals of ferrielectric CuInP${}_2$S ${}_6$ and antiferroelectric CuCrP${}_2$S${}_6$ were investigated by means of broadband dielectric spectroscopy (10 mHz–3 GHz), for several values of $x$. The phase boundary between the ferrielectric phase and the dipolar glass state was found to lie between 0.5 and 0.7, and a similar boundary between the dipolar glass state and the antiferroelectric phase is observed to be located between 0.4 and 0.2. The dipole freezing results in a broad distribution of the relaxation times. The parameters of the double-well potentials, the local polarization distribution function, and glass order parameter have been extracted from the dielectric measurements. From these results the complete phase diagram has been constructed.