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Abstract
The recent claim by Betal et al. (Phys. Chem. Chem. Phys. 2025, 27, 3150) of synthesizing air-stable Cs2CuBiBr6 double perovskite challenges the established thermodynamic instability of Cu(I)-based halide perovskites. Through rigorous reanalysis of their data, we demonstrate three critical inconsistencies: (1) X-ray diffraction patterns diverge markedly from simulated Cs2CuBiBr6 and Cs2AgBiBr6 patterns; (2) Energy-dispersive X-ray spectroscopy reveals a Cu(I):Bi(III) atomic ratio of ~2:3, violating the 1:1 stoichiometry required for A2B(I)B(III)X6 perovskites; (3) The reported bandgap (2.93 eV) exceeds that of Cs2AgBiBr6 (1.8–2.3 eV), contradicting the chemical trend predicted by density functional theory. Further, thermodynamic analysis confirms Cs2CuBiBr6’s intrinsic instability (with a large negative decomposition energy of −35 meV/atom), disfavoring its synthesis. These findings collectively invalidate Betal. et al.’s claims and underscore the profound challenges in stabilizing Cu(I)-based double perovskites, urging stringent validation of structural, compositional, and thermodynamic data in perovskite research.
