High Pressure Structural and Optical Properties of Two-Dimensional Hybrid Halide Perovskite (CH₃NH₃)₃Bi₂Br₉

Two-dimensional (2D) hybrid halide perovskite is emerging as the next generation of photoelectronic materials. Herein, a typical 2D halide perovskite of MA₃Bi₂Br₉ (MA = CH₃NH₃) is chosen for high pressure research to explore the distinct structural and property characteristics of the inorganic and organic compositions therein. Upon compression above 4.3 GPa, the distortion and tilting of inorganic BiBr₆ octahedra dominate the phase transition of MA₃Bi₂Br₉ from trigonal to monoclinic. Meanwhile, exceptionally anisotropic compressibilities are observed between intra- and interlayer structures, which originate from the unique geometry of puckered layer. In addition, the presence of organic MA⁺ cations contributes to the flexible structural nature of MA₃Bi₂Br₉. Meanwhile, the geometrical changes of inorganic components determine the relationships between structure and band gap under pressure. This work not only demonstrates the intriguing structure nature of MA₃Bi₂Br₉ but also reveals the individual contributions on the structure-property diagram from inorganic (BiBr₆ octahedra) and organic (MA cations) components.