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

We carried out a theoretical study on hexahalometallate single crystals X 2SnBr6 (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 angstrom) and Rb (1.64 angstrom). The direct band gap Gamma-Gamma of 1.272 eV and 1.1019 eV for, respectively, Cs2SnBr6 and Rb 2SnBr 6 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.