Pressure-Induced the Purification of Cs₄PbBr₆ from CsPbBr₃ Impurity

Lead halide perovskites have been of great interest due to their promising optoelectronic applications. While solution-based synthetic methods have been well developed, they often form mixtures of perovskites with several different phases, which cannot be easily purified by conventional purification methods. Here we report a high-pressure purification method of Cs₄PbBr₆ perovskite containing a trace amount of CsPbBr₃. The synthesized bulk Cs₄PbBr₆ perovskites exhibit strong photoluminescence at λ_peak = ~ 520 nm because of CsPbBr₃ embedded in Cs₄PbBr₆. Our approach utilized that CsPbBr3 crystals are sensitive to the change of pressure and make distortion of Pb-Br bonds at moderately low pressure, while Cs₄PbBr₆ crystals are much stable at high-pressure condition. With the application of high pressure (20 GPa), the crystalline CsPbBr₃ phase turns amorphous while the crystal structure of Cs₄PbBr₆ is retained without change. Almost 95% of the CsPbBr₃ phase can be removed by one cycle of compression. Cs₄PbBr₆ which originally shows strong green fluorescence becomes non-fluorescent after one cycle of compression. High-pressure compression can be used to selectively remove CsPbBr₃ by pressure-induced amorphization without depreciating the Cs₄PbBr₆ crystal structure.