Bokotey O. First-principles calculations of electrons and phonons subsystems and optical properties of Hg3X2Y2 type gyrotropic crystals

The thesis is devoted to the investigation from the first-principles the electrons and phonons subsystems and optical properties of Hg3X2Y2 type gyrotropic crystals. The comparative analysis of the anionic substitution influence on the electronic structure of cubic mercury chalcogenhalogenides was conducted. It was established that anionic Te Se S substitution in the Hg3X2Y2 compounds leads to the bandgap width increase. Using LDA+U approximation for calculations of Hg3Te2Cl2 and Hg3Se2Cl2 crystals band structure values of the indirect band gap close to experimentally determined from the analysis of optical spectra were obtained. The presence of five separated subbands formed by hybridization of s/p- states of cation and anions is the feature of electronic spectrum in the valence band of ternary Hg3X2Y2 compounds. The influence of point defects on the formation of Hg3Te2Cl2 crystal band structure using the supercell method was analyzed. It was shown that anionic vacancies modify the structure of energy bands that reflects on the electronic and optical properties of crystal under investigation. It was established that the nature of interatomic interactions in the crystals of Hg3X2Y2 type has complex character and includes ionic and covalent parts. The detailed symmetry analysis of the fundamental modes, phonons dispersion, total and partial phonons density of states for Hg3Te2Cl2 crystal was conducted. The simulation of the Raman spectrum and identification of the phonons characteristic lines to define atomic vibrations values was performed on the basis of their symmetry analysis in close comparison with results of lattice-dynamical calculations for Hg3Te2Cl2 crystal. It was found that all the optical modes are active in Raman spectra. The modes of T symmetry are active in both Raman spectra and IR-spectra due to absence of symmetry center in the studied crystal. The interpretation of experimental results of optical activity study in Hg3X2Cl2 (X = Se, Te) crystals was performed on the basis of energy electrons spectra studies. The correlation between the existence of optical activity phenomenon and realization of direct optical transitions in studied crystals was established. The gyration tensor components for Hg3Se2Cl2 crystal were calculated. Theoretical calculations of values of refractive indices, optical dielectric constants and reflection coefficients for (a), (b), (c) - Hg3S2Cl2 polymorphs and Hg3Te2Cl2 crystal within Harrison's bonding orbitals model were carried out. The calculations were performed for the spectral region far from the absorption edge, where the dispersion of refractive index is absent. It was shown that Harrison's model allows to analyze the optical properties of Hg3X2Y2 type compounds.