Exploring the Stability of Mixed-Halide Vacancy-Ordered Quadruple Perovskites

The structural and optical properties of the halide substitutions of Br– and I– into Cs4MIIMIII2Cl12 (MII = Cd2+, Mn2+; MIII = Bi3+, Sb3+) have been investigated. All compositions adopt the ⟨111⟩ layered vacancy-ordered quadruple perovskite structure with R3̅m space group symmetry. Through incremental halide substitution reactions, we show that significant bromide incorporation is possible (>25% for Cd2+-containing structures). The larger halide ions (Br– or I–) preferentially occupy the anion sites adjacent to the cation-vacancy layer. In those compositions where MII is Cd2+, incorporation of bromide ions leads to substantial MII/vacancy antisite disorder, which is accompanied by a more even distribution of bromide substitution over the two chemically distinct anion sites. The Cs4CdMIII2Cl12 compounds can incorporate over twice the amount of bromide as analogous Mn2+-containing compounds, with a maximum of 29(2)% bromide substitution found for Cs4CdSb2Cl12. Iodide incorporation is more limited, with a maximum of ∼6% halide substitution for Cs4CdBi2Cl12. The incorporation of the heavier, less electronegative Br– and I– ions results in a red shift of the onset of optical absorption in a Vegard's Law type fashion. The effect is largest for Cs4CdBi2Cl12–zXz, where the absorption onset shifts from 3.20(1) eV to 2.99(1) eV as the composition changes from Cs4CdBi2Cl12 to Cs4CdBi2Cl8.9Br3.1. As discussed in the paper, these results offer some insights into the factors that stabilize the vacancy-ordered quadruple perovskite structure.