Inter-Conversion between Different Compounds of Ternary Cs-Pb-Br System.

The perovskite CsPbBr₃ attracts great attention due to its potential in optoelectronics. However, stability remains a major obstacle to achieving its effecting application. In this work, we prepared CsPbBr₃ solids through a simple reaction and investigated reversible conversion between CsPbBr₃, Cs₄PbBr₆, and CsPb₂Br₅. We found that CsPbBr₃ can be respectively converted to Cs₄PbBr₆ or CsPb₂Br₅ by reacting with CsBr or PbBr₂. Thermodynamic analysis demonstrated that the chemical reactions above were exothermic and occurred spontaneously. Moreover, the formed Cs₄PbBr₆ could be converted to CsPbBr₃ reversely, and then progressively converted to Cs-deficient CsPb₂Br₅ by extraction of CsBr with water. The CsPb₂Br₅ was converted to CsPbBr₃ reversely under thermal annealing at 400 °C. The thermodynamic processes of these conversions between the three compounds above were clarified. Our findings regarding the conversions not only provide a new method for controlled synthesis of the ternary Cs-Pb-Br materials but also clarify the underlying mechanism for the instability of perovskites CsPbBr₃.