Anion-anion exchange effect on crystal, electronic and mechanical properties of eco-friendly lead-free Cs₂SnX₆ and CsSnX₃ halides when forming Cs₂SnX_6−xY_x and CsSnX_3−xY_x (X, Y[Formula presented]I, Br, Cl) solid solutions

We report on first-principles calculations of the electronic structure, chemical stability and mechanical properties of the lead-free perovskite CsSnX₃ and defect-perovskite Cs₂SnX₆ halides and their mixed Cs₂SnX_6-xY_x and CsSnX_3-xY_x (X, Y[Formula presented]I, Br, Cl) solid solutions. The present results reveal that, is spite of the fact that cubic perovskite structures are characteristic of Cs₂SnX₆ and CsSnX₃ halides, the mixed Cs₂SnX_6-xY_x and CsSnX_3-xY_x solid solutions are crystallized in the structures which deviate from the cubic type. When substituting iodine by bromine and, further, by chlorine in the ternary Cs₂SnX₆/CsSnX₃ and mixed Cs₂SnX_6-xY_x/CsSnX_3-xY_x (X, Y[Formula presented]I, Br, Cl) halides, the tendencies of decreasing the unit-cell parameters and the formation energy are detected. At such sequences of substitutions in the Cs₂SnX_6-xY_x and CsSnX_3-xY_x halides, they become more stable. Increasing the relative content of Br and Cl atoms favors the chemical stability in such a case. The present calculations implemented for the Cs₂SnX₆ and CsSnX₃ halides indicate that the valence X p-states are the main contributors to the valence-band region, while the unoccupied electronic p- and d-states associated with tin dominate at the bottom of the conduction band. It was obtained that X p-states in these ternary halides are hybridized with electronic states associated with cesium and tin atoms revealing the presence of the covalent component of the chemical bonding. The energy band gap of the defect-perovskite Cs₂SnX_6-xY_x halides increases significantly when substituting iodine by bromine and, then, by chlorine, while such a tendency is weaker at such substitutions in the case of CsSnX_3-xY_x solid solution. The bulk moduli B are bigger in perovskite CsSnX₃ halides in comparison with those in Cs₂SnX₆ compounds.