Stability of Cs₂NaBiBr₆ and Cs₂NaBiCl₆

Bismuth-based halide perovskites are nontoxic alternatives to widely studied lead-based perovskites for optoelectronic applications. Here, we synthesized Cs2NaBiCl6 thin films and attempted to synthesize Cs2NaBiBr6 using physical vapor deposition. While Cs2NaBiCl6 forms a stable cubic structure with a 3.4 eV band gap and could be synthesized successfully, Cs2NaBiBr6 does not form and is unstable with respect to dissociation into Cs3–xNaxBi2Br9 and Cs3–xNaxBiBr6. Furthermore, the close X-ray diffraction patterns of Cs3–xNaxBi2Br9 and Cs2NaBiBr6 raise doubts about the previous reports of the latter's formation based on X-ray diffraction alone.