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 AgInP2S6 crystal were reported. Data were analyzed within the framework of the group theory approach. Results were compared to the experimental data.

A concept of the elementary energy bands was applied to interpret the electronic energy spectra of crystals from the cubic system and the superlattices (GaAs) /(AlAs)(Si) /(Ge) built upon them. Based upon this example, it was shown that the elementary energy bands concept elaborated in the emptylattice approximation allows one to predict the symmetry and topology of the valence band of artificial periodic systems as well as the spatial valence-electron density distribution in a unit cell of those systems. A verification of this conclusion was confirmed by the ab-initio band-structure calculations of the superlattices mentioned above. An energy-structural factor was found that joins two elementary energy bands characteristic for T d 2 symmetry crystals into a physical complex that is related to the elementary energy band of the O h 7 symmetry crystals. This relationship between the elementary energy bands was utilized to find the actual Wyckoff position in the superlattice's unit cell responsible for the spatial electron density distribution.

Studies on the phases of CuInP2S6 crystal exhibiting phase transitions of the order-disorder type are performed.A possibility for the cooperative JahnTeller eect to occur in this crystal is analyzed. For this purpose, the procedure of constructing the potential energy matrix is generalized for the case of crystal and it is illustrated by the construction of the adiabatic potentials for Γ5−Γ5 vibronic coupling for the proto-and paraelectric phases of CuInP2S6. The structure of the obtained potentials is analyzed together with the discussion on the appearance of spontaneous polarization in this crystal.

The elementary energy bands concept together with the analysis of the Davydov-like splitting were applied for the first time for investigation of energy band structure and spatial electron density distribution in hexagonal crystals. This approach was used for consideration of the valence band states and for determination of peculiarities of the spatial charge density distribution in the unit cell of the layered b-InSe crystal. It was established that the valence charge density can be located in the so-called Van-der-Waals space. Conclusions following from the elementary energy bands concept were confirmed by the ab initio calculations of the band structure of b-InSe.