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Anion‐substituted lead‐free double perovskite halides through mechanochemical synthesis is realized. According to molar concentration (x), some of the Br atoms in Cs2AgBiBr6 are replaced by halogen Cl atoms, which allows us to achieve heteroanionic Cs2AgBiBr6−xClx. X‐ray diffraction (XRD) analyses show that the as‐synthesized Cs2AgBiBr6−xClx compounds undergo structural transitions with a shift of diffraction peaks attributed to changes in their lattice parameters. The chemical compositions and the optical properties of the anion‐mixed Cs2AgBiBr6−xClx compounds using energy‐dispersive X‐ray spectroscopy (EDS), UV–visible optical absorption, and photoluminescence spectroscopy are examined. It is found that the electronic bandgap in the heteroanionic Cs2AgBiBr6−xClx increases progressively with increasing Cl content and the emitted colors are blueshifted from orange to yellow color with increasing Cl content in Cs2AgBi(Br,Cl)6 powders. The results can be used to fabricate lead‐free optoelectronic devices with high performance via bandgap engineering.

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