Sublattice distortion resulting from alloying compositionally distinct double perovskites is shown to influence photoluminescence emission in Cs 2 Ag 1−x Na x BiCl 6 (0 < x < 1). The end members show negligible photoluminescence, whereas interestingly the alloys exhibit broad photoluminescence. These emissions are attributed to self-trapped excitons (STE) resulting from sublattice distortions arising due to the mismatch in [AgCl 6 ] 5− and [BiCl 6 ] 3− octahedra. Change in sublattice distortions plays significant role in the formation and recombination of STEs. The STE emission intensity and quantum yield greatly depend on x, with highest intensity observed for x = 0.75, consistent with a large change in sublattice found at this x. Variation in photoluminescence properties with composition follows a similar trend as that of bandgap and phonon vibrational changes observed due to sublattice distortion. Temperature-dependent phonon vibrations and photoluminescence studies reveal a giant electron−phonon coupling. A strong synergy between STE emissions, electron−phonon coupling, bandgap, and phonon vibrations in double perovskites with sublattice distortions is demonstrated.

Bandgap engineering in lead-free Cs2Ag1-xNaxBiCl6 (x = 0 to 1) double perovskite alloys synthesized through solution-based approach is investigated. The bandgap is shown to vary from 2.64 eV to 3.01 eV as Ag+ at B′ site gets replaced with Na+ cation. Despite a linear change in the lattice parameter according to Vegard's law, bandgap (Eg) changes in a nonlinear fashion for x = 0 to 1 with much lower Eg values observed than predicted by Vegard's rule. Further, we show the bandgap bowing effect in Cs2Ag1-xNaxBiCl6. Raman spectroscopic studies reveal that the changes in the vibrational mode positions arise due to the systematic variations in local distortions of [BiCl6]3– and [AgCl6]5– octahedra. The bandgap change, Raman mode frequency shift, Raman peak width, and the ratio of intensities of Raman modes all show a similar trend as a function of Na substitution concentration (x). The changes are minimal and linear for x from 0 to ∼0.6 and deviate sharply for higher Na concentration (x &gt; 0.6). These observations strongly suggest that the sublattice distortion in the A2B′B″X6 lattice arises due to a mismatch in the octahedra. This imparts a nonlinear change in the bandgap. Thus, a strong interplay between the [Ag(Na)Cl6]5− and [BiCl6]3– octahedra is shown to have a significant influence on the deviation of bandgap from Vegard's rule and further enforces the bandgap bowing effect in Cs2Ag1-xNaxBiCl6.

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–x Cl x (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–x Br x ]5– and [MBr6–x Cl x ]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–x Cl x 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.

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This study brings out the photoinduced thermal instability in an otherwise thermally stable Cs2AgBiBr6 double perovskite.