Bismuth-Based Halide Double Perovskite Cs2LiBiCl6: Crystal Structure, Luminescence, and Stability

A new bismuth-based halide double perovskite Cs2LiBiCl6 was isolated successfully using solid-state reactions. The crystal structure was investigated using X-ray diffraction and complemented by 7Li solid-state nuclear magnetic resonance spectroscopy, which indicated the highly ordered nature of Li and Bi ions in the B sublattice of double perovskite. Compared with the Na analogue, the more moisture-sensitive Cs2LiBiCl6 has a smaller indirect band gap of 3.15(2) eV and red-shift luminescence of around 612 nm as well as stronger intensities at Mn2+ doping. The stability mechanism of Cs2LiBiCl6 was discussed together with that of Cs2NaBiCl6 and their inaccessible Br counterparts based on the tolerance and octahedral factors and density functional theory (DFT) calculations. Owing to the small size of Li+ cations, Cs2LiBiCl6 has low tolerance and octahedral factors close to the boundary between the stable perovskite and nonperovskite compounds, which possess a small decomposition energy and thus easily coexist with other ternary phases. The Br-based compositions of Cs2LiBiBr6 and Cs2NaBiBr6 have even smaller octahedral factors and negative decomposition energies, and therefore their double perovskite polymorphs are destabilized. DFT simulations indicate that the moisture sensitivity of Cs2LiBiCl6 is related to the coordination preference of Li+ due to its smaller size and the steric effect from water molecules, in contrast with the larger Na+ showing a more stable octahedral configuration under the water incorporation. These mechanisms combining the geometry factors and decomposition energies provide new insights into the stability of halide double perovskites.