Unveiling Solvent-Related Effect on Phase Transformations in CsBr- PbBr2 System: Coordination and Ratio of Precursors

All-inorganic cesium lead halide perovskite nano-crystals have emerged as attractive optoelectronic nanomaterials owing to their stabilities and highly efficient photoluminescence. However, the inorganic perovskites of CsPbBr3 synthesized by the solution method often suffer from byproducts such as Cs4PbBr6 and CsPb2Br5. Herein, we have investigated thoroughly the solvent-related effect on the phase formation in CsBr-PbBr2 system through single crystal X-ray diffraction measurement. It is found that the prepared product is dominantly determined by the coordination number (CN) of Pb(II) and the ratio of precursors. By use of dimethyl sulfoxide (DMSO) or dimethylformamide (DMF) as the solvent, Pb2+ is found to be surrounded by six-coordination sites, and the products can be tuned from CsPbBr3 to Cs4PbBr6 by increasing the precursor ratio of CsBr to PbBr2. On the contrary, in the solvent of water, only Pb2+ eight-coordinated crystal of CsPb2Br5 can be produced, regardless of the ratio of CsBr to PbBr2. More importantly, with the investigation of the extended X-ray absorption fine structure (EXAFS) for Pb L-II edge in precursor solutions, we identify that the CNs of Pb(II) of resultants are the same as those of the corresponding plumbite oligomers in precursor solutions. In addition, the phase transitions from Cs4PbBr6 to CsPbBr3, amorphous state, and CsPb2Br5 triggered by water vapor have also been observed clearly. This work not only enriches our understanding of the phase formation of CsBr-PbBr2 system but also provides the knowledge of the degradation of halide perovskites in the environment of humidity.