Abstract

Purpose. Interest in the study of crystalline Cu₇GeS₅I caused by the presence of high ionic conductivity, which nature has not yet been proper explained. The calculation of Cu₇GeS₅I 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 first-principle calculations of energy band structure, density of states and valence charge density distribution of Cu₇GeS₅I cubic crystal performed by the density functional theory in the LDA+U-approximation.
Results. According to performed calculations, Cu₇GeS₅I is the direct-gap semiconductor with the calculated band gap width E_dg = 2.13 eV in the LDA+U-approximation. Valence band of Cu₇GeS₅I consists of four separate bunches of energy-allowed bands divided by the forbidden gaps. The analysis of partial contributions into the density of electronic states allowed to identify the genetic origin of different subbands of the valence band as well as to obtain the data of chemical bond formation in the investigated crystal. In the upper part of Cu₇GeS₅I valence band, it was found the significant hybridization of occupied d-states of copper and delocalized p-states of sulfur and iodine, which is undoubtedly associated with the covalent nature of chemical bonding between S, I and Cu atoms. Ab initio calculation results of electron density ρ(r) distribution have shown that the main charge in [GeS₄] tetrahedra is located on sulfur atoms, which preferably form the covalent bonds with germanium atoms. Corresponding contours ρ(r) are deformed along S–Ge direction, whereby this bond has the polarized ion-covalent character. Cu–S and Cu–I bonds can also be considered as an ion-covalent, but with the predominant ionic component. The main charge in the structural units formed with the participation of Cu atoms is concentrated on cooper atoms and it has the form of closed and almost spherical contours with very insignificant polarization in the direction to sulfur and iodine atoms.
Conclusion. Thus, Cu₇GeS₅I compound can be assigned as the material with Cu-ion transport conductivity mechanism by the analysis of crystal structure and valence density distribution maps

Keywords: argyrodite, electronic band structure, density of states, spatial distribution of valence charge, chemical bonding

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