Halide Mixing and Phase Segregation in Cs2AgBiX6 (X = Cl, Br, and I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy

All-inorganic double perovskites (elpasolites) are a promising potential alternatives to lead halide perovskites in optoelectronic applications. Although halide mixing is a well-established strategy for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single- and mixed-halide Cs2AgBiX6 (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using Cs-133 solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible within a narrow range of halide ratios (<3 mol % I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol %. Our study shows that solid-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure-activity relationships, halide mixing, and phase segregation phenomena in Cs2AgBiX6 (X = Cl, Br, and I) double perovskites.