Comment on “Air-stable double halide perovskite Cs₂CuBiBr₆: synthesis and memristor application” by A. Betal, A. Chetia, D. Saikia, K. Karmakar, G. Bera, N. V. Dambhare, A. K. Rath and S. Sahu, Phys. Chem. Chem. Phys., 2025, 27, 3150

The recent claim by Betal et al. (Phys. Chem. Chem. Phys., 2025, 27, 3150) of synthesizing the 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; and (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 per atom), disfavoring its synthesis. These findings bring into question 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.