Unexpected bowing band evolution in an all-inorganic CsSn_{1- x} Pb _x Br₃ perovskite

We theoretically investigated the structural and electronic properties of the all-inorganic perovskite CsSn_1-x Pb _x Br₃, compared with the mixed perovskite compound MA _y Cs_1-y Sn_1-x Pb _x Br₃, based on first-principle calculations. It has been demonstrated that Pb and Sn atoms are inclined to occupy the lattice sites uniformly in the all-inorganic perovskite, and this is distinguished from the most stable configurations observed in the mixed Cs-MA system. It is interesting that small Sn atoms prefer to stay close to the large MA⁺ cations, leading to smaller local structural distortion. Through spin-orbital coupling calculations, we found non-linear bowing band evolution in the all-inorganic mixed Sn-Pb system with a small bowing parameter (b = 0.35), while the band gap of MA _y Cs_1-y Sn_1-x Pb _x Br₃ was clearly reduced as the ratio of MA was around 0.5 (y ≥ 0.25). We determined the bowing band evolution in the mixed cation perovskites and the intrinsic electronic deficiency of the all-inorganic perovskite to obtain the optimal band gap.