Abstract

Purpose. Interest to the study of Ag₂S and Ag₂GeS₃ crystals is caused by the presence of high ionic conductivity which nature has not yet been adequately explained. Electronic structure calculations for these compounds are very useful in this regard and their knowledge will help to establish the character of chemical bonding between Ag and S as well as between Ge and S which is necessary for the better understanding of the ionic conductivity mechanism.
Methods. This paper presents the calculations of band structure, total and partial densities of states, spatial distribution of valence charge density for Ag₂S and Ag₂GeS₃ crystals performed within density functional theory in the local density approximation (LDA and LDA+U).
Results. It follows from the electronic structure calculations that both crystals are the directgap semiconductors with the band gap widths E_gd = 0.91 eV for Ag₂S and E_gd = 1.96 eV for Ag₂GeS₃ calculated in the LDA+U-approximation. The valence band of Ag₂GeS₃ contains four energetically isolated bunches of occupied bands. Two quasi-core bands in the valence band bottom are formed mainly by sulfur 3s-states with the insignificant impurities of Ge4s-states into the lowest subband and Ge4p-states into the next subband. The second bunch of occupied bands is formed by hybridized Ge4s- and S3p-states. Most difficult is the upper valence subband. Spectrum of density of electronic states N(E) in this region is formed by S3р- and Ag4d-atomic orbitals with the insignificant impurity of Ge4p-states. The results of ab initio of ρ(r) electronic density calculations showed that the main charge in Ag₂S is concentrated on silver atoms and Ag–S bond is predominantly ionic. The nature of ρ(r) contours in Ag₂GeS₃ crystal shows the ion-covalent bond type, while Ge–S bond is stronger than Ag–S.
Conclusion. From the analysis of crystal structure and valence density distribution maps of Ag₂GeS₃ it follows that this compound can be attributed to the compounds with Ag-ion transport along channels between the infinite chains formed by corner-sharing [GeS₄] tetrahedra

Keywords: electronic band structure, density of states, silver sulfide, silver thiogermanate, distribution of the valence charge

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