STUDY OF ELECTRONIC STRUCTURE OF Ag2Se, Sb2Se3, AgSbP2Se6

Abstract

The crystal structure of the AgSbP2Se6 compound was studied by X-ray powder method. Crystal chemical structural calculations using the EXPO–CCP14 software complex showed that the AgSbP2Se6 compound crystallizes in trigonal syngoni, SG R-3, unit cell parameters a = 6.61573 Å, c = 39.86223 Å, V=1510.94 Å3, Z=6. A crystal-chemical analysis of the structures of complex selenides Ag2Se, Sb2Se3, AgSbP2Se6 was carried out. Ag2Se compound (low-temperature modification) crystallizes in orthorhombic syngonia (P212121). Ag+ cations are bound in a 3-coordinate geometry with three equivalent Se2– atoms. Se2– anions coordinate six Ag+ around themselves. Sb2Se3 crystallizes in orthorhombic syngonia (Pnma) with a weakly expressed layered 2D-structure. Sb3+ cations are in two non-equivalent positions: in the first they form deformed [SbSe6] octahedra, in the second [SbSe5] square pyramids. The crystal structure of the compound AgSbP2Se6 (R-3) is characterized by the formation of layers from the anionic group of atoms [P2Se6]4–. In the space between the anionic groups, cations Ag+, Sb3+, which are in octahedral coordination with Se2–, are placed alternately. Sb3+ cations are located on the same plane as the P–P bond centers of [P2Se6]4– anionic groups, Ag+ cations are slightly displaced relative to this plane. The study of the optical absorption spectra of AgSbP2Se6 single crystal showed that it is characterized by a direct-band allowed type of conductivity, the band gap is Eg=1.49 eV (Tautz method based on (αhν)2=f(hν) Eg=1.48 eV). Ab initio quantum-chemical calculations of the electronic structure using the Quantum Espresso (QE) software package based on density functional theory (DFT) showed that the complex selenides Ag2Se, Sb2Se3, AgSbP2Se6 are characterized by a direct band type of conductivity, the width of the optical band gap Eg = 0.05 eV (Ag2Se ), Eg = 0.87 eV (Sb2Se3), Eg = 1.42 eV (AgSbP2Se6), Fermi energy EFermi=9.75 eV (Ag2Se), EFermi=6.12 eV (Sb2Se3), EFermi=5.34 eV (AgSbP2Se6). The optical band gap (Eg) of Sb2Se3, AgSbP2Se6 compounds are formed by electron transitions Se 4p → Sb 5p, for the Ag2Se compound Se 4p → Ag 5s.