Anionic Alloying in Hybrid Halide Cs2AgBiBr6-XClX Double Perovskites: Is it True Alloying or Preferential Occupation of Halide Ions in MX6 Octahedra?

Anionic alloying (halide mixing) in lead-free halide double perovskites is an effective strategy to tailor the optoelectronic properties including band gap. An important question that needs to be addressed is whether halide ions mix up homogeneously at the atomic scale as has already been inferred in a hybrid halide solid solution. Here, we show from Raman spectral analyses that halide ions (X = Cl-, Br-) preferentially form Br-rich or Cl-rich octahedra in Cs2AgBiBr6-xClx (x = 0 to 6; M = Ag, Bi) double perovskites. Octahedral vibrations show discontinuity in Raman shifts upon alloying, and the observation of octahedral modes from both [MCl6-xBrx]5- and [MBr6-xClx]5- with a shift from the end-member vibrational frequencies confirms the absence of homogeneous mixing (i.e., octahedra [MBr3Cl3]5-) and preferential formation of X-rich octahedra. The lattice parameter and the optical band gap of Cs2AgBiBr6-xClx vary linearly resembling Vegard's rule, suggesting a macroscopic solid solution behavior while maintaining, at the sublattice level, the preferential X-rich octahedra. This is further corroborated through comprehensive first-principles calculations that the alloyed structure with preferential occupation of halide ions, instead of local phase segregation or homogeneous mixing, tends to be the more stable configuration. An equal number of dissimilar halogen atoms in each octahedron as conventionally assumed is not a stable configuration. The linear variation of the band gap is attributed to the fact that individual Ag-X and Bi-X interactions add up to form the electronic structure, and therefore, the band gap is primarily correlated to the concentration of Cl and Br anions rather than their distribution in the individual octahedra.