An analysis of the P2S6 cluster electronic structure and its comparison with the crystal valence band in the paraelectric and ferroelectric phases has been done by first-principles calculations for Sn2P2S6 ferroelectrics. The origin of ferroelectricity has been outlined. It was established that the spontaneous polarization follows from the stereochemical activity of the electron lone pair of tin cations, which is determined by hybridization with P2S6 molecular orbitals. The chemical bonds covalence increase and rearrangement are related to the valence band changes at transition from the paraelectric phase to the ferroelectric phase.

First principles calculations of the structural, electronic and vibrational properties of the AgInP₂S₆crystal were reported. Data were analyzed within the framework of the group theory approach. Results were compared to the experimental data.

Thermoelectric properties of Sn₂P₂S₆ (SPS) ferroelectric-semiconductor crystals confirm their hole-type conductivity at room temperature which inverses to the electron-type conductivity at heating to 500 K. Their dielectric susceptibility temperature anomaly near second order phase transition at 337 K and dielectric losses in paraelectric phase both strongly depend on the deviation from the stoichiometric content. Calculated in LDA approach donor energy level of the sulfur vacancies and acceptor level of the tin vacancies are placed near the top of valence band. Such position in the energy gap of donor states mostly determines the semiconductive properties of SPS crystals, and they is related to strong hybridization between sp² hybrid orbitals of phosphorus atoms and p orbitals of sulfur atoms inside anions (P₂S₅)⁴⁻ that contain sulfur vacancy.

Layered CuInP ${}_2$S(Se) ${}_6$ crystals in ferrielectric and paraelectric phases were investigated for the first time via the plane-wave pseudo-potential methodology within density functional theory (DFT). Was proved that structural and electronic properties of the van der Waals layered crystals should be calculated applying dispersion correction to the DFT formalism. The partial and projected density of states were used to evaluate the electronic band structure changes under phase transition. Instability of the CuInP ${}_2$S(Se) ${}_6$ crystals was successfully explained by the second-order Jahn-Teller effect.

The Raman scattering spectra for the MnPS₃ single crystal have been studied at frequencies ranging from 5 to 1200 cm⁻¹, for temperatures ranging from 5 to 295 K. This is the first time that studies of low-temperature polarized Raman spectra in a magnetically ordered phase are presented for MnPS₃. Energy splitting between some of A_g- and B_g-modes, which were previously believed to be energy equivalent in crystals from this family, has been found. The nature of the additional phonon lines observed in the Raman scattering spectra is discussed. It is found that at low temperatures, the magnetic scattering spectrum has a complex form that is related to the presence of both two-magnon and a phonon-magnon bands. The vibrational properties of the MnPS₃ crystal were calculated using the LDA approximation with the DFT-D (OBS) dispersion correction implemented in the CASTEP computation package. Phonon dispersion curves and the total and partial phonon densities of states have been calculated in the antiferromagnetic phase of the MnPS₃ crystal. The calculated results are in good agreement with the experimental Raman spectra.

The first-principles calculations in LDA approach were performed for Sn₂P₂S₆ ferroelectrics. The evolution of energy band structure and charge density distribution clearly illustrate the stereoactivity of the tin atoms lone pair electrons and their leading role in the phase transition nature which is described by three-well potential. Energy levels and charge density distribution were calculated for the tin vacancies acceptor states and for the sulfur vacancies donor states in relaxed structure of the paraelectric phase. Taking into account the named donors and acceptors, small electron polarons at Sn⁺ cations and small hole polarons at S⁻ anions, the relationship of thermally induced lattice multistability with light induced optical bistability for Sn₂P₂S₆ ferroelectrics-semiconductors was established.