Pressure‐Induced Emission (PIE) and Phase Transition of a Two‐dimensional Halide Double Perovskite (BA) 4 AgBiBr 8 (BA=CH 3 (CH 2 ) 3 NH 3 + )

Two‐dimensional (2D) halide perovskites have attracted significant attention due to their compositional flexibility and electronic diversity. Understanding the structure–property relationships in 2D double perovskites is essential for their development for optoelectronic applications. In this work, we observed the emergence of pressure‐induced emission (PIE) at 2.5 GPa with a broad emission band and large Stokes shift from initially nonfluorescent (BA) 4 AgBiBr 8 (BA=CH 3 (CH 2 ) 3 NH 3 + ). The emission intensity increased significantly upon further compression up to 8.2 GPa. Moreover, the band gap narrowed from the starting 2.61 eV to 2.19 eV at 25.0 GPa accompanied by a color change from light yellow to dark yellow. Analysis of combined in situ high‐pressure photoluminescence, absorption, and angle‐dispersive X‐ray diffraction data indicates that the observed PIE can be attributed to the emission from self‐trapped excitons. This coincides with [AgBr 6 ] 5− and [BiBr 6 ] 3− inter‐octahedral tilting which cause a structural phase transition. High‐pressure study on (BA) 4 AgBiBr 8 sheds light on the relationship between the structure and optical properties that may improve the material's potential applications in the fields of pressure sensing, information storage and trademark security.