Unsubstantiated synthesis of Cs₂CuBiCl₆ double perovskite

Halide perovskites have revolutionized optoelectronics due to their tunable bandgaps, high absorption coefficients, and ease of fabrication. Double perovskites, with their general formula A₂B(I)B(III)X₆, offer additional compositional flexibility and stability, making them attractive for applications such as light-emitting diodes and photodetectors. However, the synthesis of Cu(I)-based double perovskites remains challenging due to the thermodynamic instability arising from the high energy of Cu 3 d¹⁰ orbitals, which favor tetrahedral coordination over the octahedral coordination required for double perovskites. Recently, Neelu et al. reported the synthesis of Cs₂CuBiCl₆, claiming it as the first successful synthesis of a Cu(I)-based double perovskite [Opt. Mater. 143 (2023) 114250]. However, a critical reevaluation of their data reveals inconsistencies that challenge this claim. This comment highlights the lack of evidence supporting the synthesis of Cs₂CuBiCl₆ and underscores the challenges associated with synthesizing thermodynamically unstable Cu(I)-based double perovskites.