Vakulchak V. The electronic structure of silicon and germanium dichalcogenides and superionics M2Si(Ge)S3 (M = Li, Na, Ag)

The thesis is devoted to the investigation from the first principles the electronic structure, the chemical bond and optical properties of different polymorphic phases of silicon and germanium dichalcogenides, silicon telluride Si2Te3 and superionics of MI2AIVS3 (M = Li, Na, Ag; АIV = Si, Ge) type. The comparative analysis of the influence of isocationic and isoanionic substitution in AIVBVI2 crystals on the electronic structure of the present compounds was carried out. It is established that anionic S-Se and cationic Si-Ge substitution in AIVBVI2 binary compounds leads to a decrease of the total width of the valence band and the band gap. The feature of electronic spectrum in the valence band of binary AIVBVI2 and ternary MI2AIVS3 compounds is the presence of three separated subbands formed by hybridization of s - and p- states of anion and cation. A comprehensive study of the electrical conductivity, absorption edge and photoconductivity spectra of Si2Te3 layered crystals was performed in a wide temperature range. It was revealed that the Urbach form of the absorption edge in crystalline Si2Te3 due to a strong electron-phonon interaction. From the analysis of the crystal structure and a spatial distribution of the electron charge density it follows that in Li2SiS3 and Na2GeS3 superionics in covalently bound skeleton of [SiS4] and [GeS4] tetrahedra there are "tunnels" with minimal electron density, which serve as channels for moving of alkali metals ions, and this is caused their high ionic conductivity.