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Reversible Release and Fixation of Bromine in Vacancy‐Ordered Bromide Perovskites

Bromine is an irreplaceable reagent for many important applications including necessary additive for flame retardants and rubber treatment. However, the toxicity, causticity, and vaporability of bromine make the safe usage and transportation of bromine challenging, which can be alleviated by the bromine prefixation. Herein, vacancy‐ordered bromide perovskites such as Cs4Sb2Br12 and Cs2PdBr6 are selected out for the bromine fixation through density‐functional theory calculations and experiments. The redox within the bromide perovskites is designed to provide the driving force for the reversible release and fixation of bromine. Heating and cooling the title bromide perovskites in a necked ampoule make the bromine visible and separable from the solid residue. The composition engineering (i.e., Cs4Sb2Br12 vs Rb4Sb2Br12, and Cs4Sb2Br12 vs Cs2PdBr6) can effectively tune the stability (against water and high temperature) and the bromine storage capacity (from 9.30% to 18.76%). In addition, this work can provide a one‐step preparation method for the liquid bromine without further purification.