Electronic structure and chemical bonding of Cu<sub>7</sub>GeS<sub>5</sub>I superionic conductor
DOI:
https://doi.org/10.24144/2415-8038.2017.41.41-50Keywords:
argyrodite, electronic band structure, density of states, spatial distribution of valence charge, chemical bondingAbstract
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 Cu7GeS5Ielectronic structure can be very useful in this regard, its know-ledge 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 Edg = 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 r(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 r(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.
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