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 Cs_3-xNa_xBi₂Br₉ and Cs_3-xNa_xBiBr₆. Furthermore, the close X-ray diffraction patterns of Cs_3-xNa_xBi₂Br₉ and Cs₂NaBiBr₆ raise doubts about the previous reports of the latter's formation based on X-ray diffraction alone.