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.