Tunable Perovskite-Derived Bismuth Halides: Cs₃Bi₂(Cl_1–<i>x</i>I_<i>x</i>)₉

Abstract

Bismuth-based\nperovskites are of interest as safer alternatives\nto lead-based optoelectronic materials. Prior studies have reported\non the compounds Cs₃Bi₂Cl₉, Cs₃Bi₂I₉, and Cs₃Bi₂Cl₃I₆. Here we examine a range of compounds\nof the formula Cs₃Bi₂(Cl_1–<i>x</i>I_<i>x</i>)₉, where <i>x</i> takes values from 0.09 to 0.52. Powder and single-crystal\nX-ray diffraction were used to determine that all of these compounds\nadopt the layered vacancy-ordered perovskite structure observed for\nCs₃Bi₂Cl₃I₆, which is\nalso the high-temperature phase of Cs₃Bi₂Cl₉. We find that, even with very small iodine incorporation,\nthe structure is switched to that of Cs₃Bi₂Cl₃I₆, with I atoms displaying a distinct preference\nfor the capping sites on the BiX₆ octahedra. Optical absorption\nspectroscopy was employed to study the evolution of optical properties\nof these materials, and this is complemented by density functional\ntheory electronic structure calculations. Three main absorption features\nwere observed for these compounds, and with increasing <i>x</i>, the lowest-energy features are red-shifted.