Pressure-induced structural transition and band gap evolution of double perovskite Cs₂AgBiBr₆ nanocrystals

Lead-free double halide perovskite nanocrystals (NCs) are attracting increasing attention due to their non-toxic nature and exceptional stability as a substitute material for lead-based perovskites. Herein, we investigate the relationship between the structural and optical properties of double halide perovskite Cs2AgBiBr6 NCs under high pressure. In situ synchrotron high-pressure powder X-ray diffraction and Raman experiments indicated that the structure of Cs2AgBiBr6 NCs transformed into a tetragonal from a cubic system at 2.3 GPa. Pressure-dependent absorption demonstrated that the band gap changes in the sequence red-shift → blue-shift. First-principles calculations further indicated that the band gap evolution was highly related to the orbital interactions, associated with the tilting and distortion of [AgBr6]5- and [BiBr6]3- octahedra under pressure. It is worth noting that the quenched absorption peak of Cs2AgBiBr6 NCs was slightly blue-shifted compared with that of the initial one under ambient conditions, which is in stark contrast to that of the corresponding bulk counterparts. This is because the structure of the sample was not yet recovered and maintained a certain degree of distortion after fully releasing the pressure. What's more, the NCs after decompression are a mixture of cubic and tetragonal phases, which leads to a larger quenched band gap than that of the initial value. Our results improve the understanding of the structural and optical properties of nanostructured double halide perovskites, thus providing a basis for their application in optoelectronic devices.