Structural, Elastic, Electronic and Optical Properties Study of Hexahalometallate Single Crystals X2SnBr6 (X = Rb, Cs)

Abstract

We carried out a theoretical study on hexahalometallate single crystals X₂SnBr₆ (X = Rb, Cs) with the density functional theory framework, using generalized gradient approximation of the Perdew, Burke, and Ernzerhof and Heyd-Scuseria-Ernzerhof hybrid functional. The lattice constant of the compounds Cs₂SnBr₆ and Rb₂SnBr₆ is consistent with the experimental data currently available, with an uncertainty of 0.04% and 0.07%, respectively. For the first time, the elastic moduli of Cs₂SnBr₆ at equilibrium and under pressure effect were estimated. When fitted to the Birch-Murnaghan equation of state, the bulk modulus of Cs₂SnBr₆ and Rb₂SnBr₆ obtained using the generalized gradient approximation optimization is closer to the experimental one. The interatomic distances of Cs₂SnBr₆ are greater than those of Rb₂SnBr₆, because these distances are proportional to the radius of Cs (1.73 Å) and Rb (1.64 Å). The direct band gap Γ-Γ of 1.272 eV and 1.1019 eV for, respectively, Cs₂SnBr₆ and Rb₂SnBr₆ explains their semiconducting character. The Sn-s state dominates in the conduction band minima, indicating that Sn and Br are in ionic connection in both Cs₂SnBr₆ and Rb₂SnBr₆.