Stability of Cs 2 NaBiBr 6 and Cs 2 NaBiCl 6 .

Bismuth-based halide perovskites are nontoxic alternatives to widely studied lead-based perovskites for optoelectronic applications. Here, we synthesized Cs 2 NaBiCl 6 thin films and attempted to synthesize Cs 2 NaBiBr 6 using physical vapor deposition. While Cs 2 NaBiCl 6 forms a stable cubic structure with a 3.4 eV band gap and could be synthesized successfully, Cs 2 NaBiBr 6 does not form and is unstable with respect to dissociation into Cs 3- x Na x Bi 2 Br 9 and Cs 3- x Na x BiBr 6 . Furthermore, the close X-ray diffraction patterns of Cs 3- x Na x Bi 2 Br 9 and Cs 2 NaBiBr 6 raise doubts about the previous reports of the latter's formation based on X-ray diffraction alone.