Solovyov M. Transformation of the energy of electron, exciton and phonon excitations in A4BX6 group crystals.

The work is devoted to the systematic experimental and first principles investigation of electronic, phonon, opticaland electrical properties of A4BX6 group crystals.The synthesis of the investigated compound is performed by the vertical Bridgmantechnique. The high-frequency binary compounds TlI and HgI2 are used as the input agents. The initial components were taken according to equi-molar ratios. Preliminary purification of the salts was performed using reiterated recrystallisation from the melt in quartz ampoules, and a vacuum sublimation.The structure data, atomic coordinates and interatomic distances calculation are based on experimental X-ray diffraction data.The energy band structure of Tl4CdI6 and Tl4HgI6 is calculated from the first principles within the generalized gradient approximation (GGA). The band structure and density of states were calculated using a pseudopotential method in the framework of density functional theory. The effective mass of an electron and a hole have been determined using the first-principle calculations of band energy structure. The experimental and theoretical results are in good agreement.Structure data and atomic coordinates have been obtained by Rietveld method. The compounds of Tl4HgI6 and Tl4CdI6 are isomorphs with each other and crystallize in a tetragonal lattice with the space group of P4/mnc. Present the results of theoretical calculations of phonon spectra of A4BX6 group crystals. Based on group theory analysis the symmetry classification of phonon modes carried out. Moreover the distribution of vibrations for symmetry classes of single crystals and the selection rules for vibrations in the IR- and Raman spectra are obtained.The results of the infrared and Raman spectra of Tl4HgI6 and Tl4CdI6 crystals are present. On the basis of a factor group analysis and phonon frequencies calculated for Tl4HgI6 and Tl4CdI6, the relations between the vibrational spectra and the crystal structures have been determined. Moreover, the distribution of vibrations for symmetry classes of single crystals and the selection rules for vibrations in the infrared absorption and Raman spectra are obtained. Phonon modes frequencies and their origin determination are based on Raman scattering data.