Tunable Perovskite-Derived Bismuth Halides: Cs₃Bi₂(Cl_1-I)₉

Bismuth-based perovskites are of interest as safer alternatives to lead-based optoelectronic materials. Prior studies have reported on the compounds Cs₃Bi₂Cl₉, Cs₃Bi₂I₉, and Cs₃Bi₂Cl₃I₆. Here we examine a range of compounds of the formula Cs₃Bi₂(Cl_1-I)₉, where x takes values from 0.09 to 0.52. Powder and single-crystal X-ray diffraction were used to determine that all of these compounds adopt the layered vacancy-ordered perovskite structure observed for Cs₃Bi₂Cl₃I₆, which is also the high-temperature phase of Cs₃Bi₂Cl₉. We find that, even with very small iodine incorporation, the structure is switched to that of Cs₃Bi₂Cl₃I₆, with I atoms displaying a distinct preference for the capping sites on the BiX₆ octahedra. Optical absorption spectroscopy was employed to study the evolution of optical properties of these materials, and this is complemented by density functional theory electronic structure calculations. Three main absorption features were observed for these compounds, and with increasing x, the lowest-energy features are red-shifted.