ЭНЕРГЕТИЧЕСКАЯ ЗОННАЯ СТРУКТУРА, ОПТИЧЕСКИЕ СВОЙСТВА И ХИМИЧЕСКАЯ СВЯЗЬ КРИСТАЛЛА Cu7SiS5I

Abstract

Первопринципным методом теории функционала плотности (DFT) в приближении LDA+U проведены расчеты зонной структуры, полной и парциальных плотностей электронных состояний, пространственного распределения плотности электронного заряда и оптических функций: диэлектрической проницаемости, показателя преломления, коэффициентов отражения и поглощения кристалла Cu7SiS5I. По результатам расчета Cu7SiS5I является прямозонным полупроводником с рассчитанной шириной запрещенной зоны calc Egd = 2.34 эВ, близкой к экспериментально измеренной opt Eg = 2.25 эВ. Ключевые слова: аргиродит, электронная структура, плотность состояний, распределение электронной плотности, оптические функции.

Purpose: Interest in the study of crystalline Cu7SiS5I caused by the presence of high ionic conductivity, which nature has not yet been proper explained. The calculation of Cu7SiS5I electronic structure can be very useful in this regard, its knowledge will help to establish the nature of chemical bonding between Cu and S(I), which is necessary for the better understanding of ionic conductivity mechanism. Methods: This paper presents the results of calculations of the band structure, the state density distribution of the electron density and such optical functions as real 1 and imaginary 2 part of the dielectric permittivity, reflectivity R, absorption coefficient  and refractive index n of Cu7SiS5I crystal. Results: The total valence band with width of 13.85 eV for Cu7SiS5I consists from four subbands separated by forbidding intervals. The analysis of partial contributions into the density of electronic states allowed to identify the genetic origin of different subbands of the valence band, and also to obtain the formation of a chemical bond in the crystal under study. The most important feature of the electronic spectrum of the Cu7SiS5I crystal is the presence of an intense peak in the density of states N(E) formed by Cu3d-like zone. The calculated spectral dependences of the absorption and reflection coefficients show the main features of the 2(Е) dependence, and the maxima of the energy distribution of the refractive index practically coincide in position with the centers of gravity of the bands of the real part of the complex permittivity. The electron density (r) is calculated as well as the maps of the spatial distribution of charge of the valence electrons in the tetrahedron [SiS4], triangle [CuS3] and bipyramid [Cu2S3I2] are plotted. Conclusion: According to charge density of valence electrons, calculated using pseudo waves functions, the features of the chemical bond in the crystal Cu7SiS5I are studied. The observed features of the optical functions of Cu7SiS5I are identified taking into account the nature of the electronic states. Analysis of the distribution of electron density indicates that a mixed ion-covalent type of chemical bond is realized in this compound. The ion component is due to the transfer of the charge density between cations and anions due to the difference in their electronegativities. Keywords: argyrodite, electronic band structure, density of states, spatial distribution of valence charge, optical functions. PACS NUMBER: 31.10.+z, 71.15.Mb, 71.20.-b, 74.20.Pq