Energy band structure, optical properties and chemical bonding of Cu<sub>7</sub>Sis<sub>5</sub>I crystal

Abstract

Purpose: Interest in the study of crystalline Cu 7 SiS 5 I caused by the presence of high ionic conductivity, which nature has not yet been proper explained. The calculation of Cu 7 SiS 5 I 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 e 1 and imaginary e 2 part of the dielectric permittivity, reflectivity R , absorption coefficient a and refractive index n of Cu 7 SiS 5 I crystal. Results: The total valence band with width of 13.85 eV for Cu 7 SiS 5 I 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 Cu 7 SiS 5 I crystal is the presence of an intense peak in the density of states N ( E ) formed by Cu3 d- like zone. The calculated spectral dependences of the absorption and reflection coefficients show the main features of the e 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( r ) is calculated as well as the maps of the spatial distribution of charge of the valence electrons in the tetrahedron [SiS 4 ], triangle [CuS 3 ] and bipyramid [Cu 2 S 3 I 2 ] are plotted. Conclusion: According to charge density of valence electrons, calculated using pseudo waves functions, the features of the chemical bond in the crystal Cu 7 SiS 5 I are studied. The observed features of the optical functions of Cu 7 SiS 5 I 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.