The effect of divalent ions on chemical stability of eco-friendly α, γ and δ phases of CsSnI₃

In the present work, based on possibilities of density functional theory (DFT), we explore the ability of cesium tin triiodide to incorporate divalent ions (Mg²⁺, Ni²⁺, Zn²⁺, and Cd²⁺) depending on its crystal structure (α, γ and δ phases of CsSnI₃). The total energies of the α-, γ- and δ-phases of CsSnI₃ alloyed with the divalent ions are found to be negative and they do not alter essentially when changing the phase. Calculations of the formation energies indicate that, independently of the phase of CsSnI₃, the incorporating abilities for the divalent ions increases in the sequence Cd²⁺ → Mg²⁺ → Zn²⁺ → Ni²⁺. Therefore, incorporation of such divalent ions favors the chemical stability of CsSnI₃ that is very important for practical application of this iodide. We do not detect significant changes in densities of states in the sequence α-CsSnI₃ → γ-CsSnI₃ → δ-CsSnI₃. The Cs–I and Sn–I bonds in the α-, γ- and δ-phases of CsSnI₃ reveal the existence of a substantial covalent component (in addition to ionic component) of the chemical bonding. The electronic states associated with the embedding divalent ions form additional electronic states in the energy band gaps of the α-, γ- and δ-phases of CsSnI₃. Such additional electronic states associated with the divalent ions cause essential decreasing the E_g values of the α-, γ- and δ-phases of CsSnI₃ leading in some cases to metallic behavior of α-CsSnI₃. The tendency of increasing chemical stability of the α-, γ- and δ-phases of CsSnI₃ with decreasing ionic radius of the embedding divalent ion has been detected.