Octahedral Distortion and Excitonic Behavior of Cs₃Bi₂Br₉ Halide Perovskite at Low Temperature

The metal halide ionic octahedron, represented as [MX<sub>6</sub>]<sup>$n$-</sup> (M = metal cation, X = halide anion), serves as the basic structural unit in halide perovskites and plays a crucial role in determining their optoelectronic and chemical properties. Thus, it is possible to correlate the responses of metal halide perovskites to various environmental stimuli with the dynamic behaviors of the [MX<sub>6</sub>]<sup>$n$-</sup> octahedra. In this study, with the temperature-dependent single-crystal X ray diffraction (SCXRD) measurements on Cs<sub>3</sub>Bi<sub>2</sub>Br<sub>9</sub> 2D halide perovskites, we can identify two classes of distortions through the lowering of temperature: intraoctahedral distortion, which is the off-centering of Bi<sup>3+</sup> cation within a [BiBr<sub>6</sub>]<sup>3–</sup> octahedron due to the Bi<sup>3+</sup> 6s<sup>2</sup> lone pair electrons, and interoctahedral distortion, which is the collective misalignments among the [BiBr<sub>6</sub>]<sup>3–</sup> building blocks. Free exciton (FE) and self-trapped exciton (STE) models are used to study the relationship between the distortion of octahedra in Cs<sub>3</sub>Bi<sub>2</sub>Br<sub>9</sub> and the corresponding changes in its optoelectronic properties, which transform from dominating blue emission above 100 K to red emission at 4 K. In conclusion, this work provides new insights into the excitonic behaviors of perovskites and suggests a possibility that we can design and rationalize the optical properties of halide perovskites by regulating the environmental stimuli based on the knowledge of behaviors of the individual [MX<sub>6</sub>]<sup>$n$-</sup> building blocks.