Abstract

Purpose. Interest in the study of crystalline and glassy Ag₂SiS₃ caused by the presence of high ionic conductivity, the nature of which has been not proper explained. Very useful in this regard can be the calculation of Ag₂SiS₃ electronic structure, knowledge of which will help to establish the nature of the chemical bonding between Ag and S, which is necessary for a better understanding of the glass local structure and the mechanism of ion conductivity.
Methods. In this paper the first principle calculations of the energy band structure, density of states and the charge density distribution of the valence electrons of Ag₂SiS₃ monoclinic crystal were performed by the method of density functional in LDA and LDA+U-approximations.
Results. From calculations of the electronic structure follows that this is an indirect-gap semiconductor with calculated in LDA+U-approaching band gap E_gi = 2.55 eV. Ag₂SiS₃ valence band contains four separate bundles of energy-allowed areas. Two of quasi core subzones forming the bottom of the valence band, formed mostly by 3s-states of sulfur with small amount of 3s-states of silicon into the lowest sub-band and 3s-,3p-states into the subsequent sub-band. The second bundle of valence bands formed by hybridized Si3s- S3p-states. The most complicate is the upper valence sub-band. At this region the electronic density of states N(E) is formed and S3s- Ag4d-atomic orbitals with a small admixture of Si3p-states. The results of ab initio calculations of electron density ρ(r) showed that the main charge in [SiS₄] tetrahedra is located on sulfur atoms, which form preferably a covalent bond with silicon atom. Corresponding contour ρ(r) deformed in the direction of S–Si, whereby this bond is of polarized ion-covalent character. The Ag–S bond can also be considered as an ion-covalent, but with predominant ionic component.
Conclusion. Thus, from the analysis of the crystal structures and the valence density distribution maps of Ag₂SiS₃, by conductivity this compound can be related to materials with Ag-ion transport

Keywords: electronic band structure, density of states, density functional theory, spatial distribution of the valence charge, chemical bonding

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