We investigate an influence of the crystal structure imperfections on the electronic properties and dielectric functions of the In 0.5 Tl 0.5 I semiconductor in the frame of the density functional theory calculations. The tensor of electron effective mass m à ij for the InI, In 0.5 Tl 0.5 I and TlI crystals has been calculated for the valence and conduction bands at different K-points of the Brillouin zone. The dielectric functions e(hm) of the imperfect crystals based on In 0.5 Tl 0.5 I solid state solution with iodine vacancy and a thallium interstitial atom were calculated taking into consideration the inter-band and intra-band electron transitions. The studies of the imperfect crystals reveal increased low-frequency and stationary electron conductivity with anisotropy resulted from the anisotropy of the electron effective mass tensor. Our findings explain the origin of crucial changes in the band structure by formation of the donor half-occupied levels close to the unoccupied conduction bands due to the crystal structure defects, i.e., iodine vacancy or a thallium interstitial atom. It has been shown that in the case of real crystals, in particular metal-halides, the proper consideration of defects in quantum-chemical calculations results in a better matching with experimental data and, opposite to the perfect structure calculations, gives opportunities to explain the observed phenomena.

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Results of the ab initio molecular dynamics calculations of silicon crystals are presented by means of analysis of the velocity autocorrelation function and determination of mean phonon relaxation time. The mean phonon relaxation time is crucial for prediction of the phonon-associated coefficient of thermal conductivity of materials. A clear correlation between the velocity autocorrelation function relaxation time and the coefficient of thermal diffusivity has been found. The analysis of the results obtained has indicated a decrease of the velocity autocorrelation function relaxation time t with increase of temperature. The method proposed may be used to estimate the coefficient of thermal diffusivity and thermal conductivity of the materials based on silicon and of other wide-bandgap semiconductors. The correlation between kinetic energy fluctuations and relaxation time of the velocity autocorrelation function has been calculated with the relatively high coefficient of determination R 2 = 0.9396. The correlation obtained and the corresponding approach substantiate the use of kinetic energy fluctuations for the calculation of values related to heat conductivity in silicon-based semiconductors (coefficients of thermal conductivity and diffusivity).