Atomistic Insights into Halide Double Perovskite Nanocrystals obtained by Multistep Synthesis and Efficient Compositional Engineering

Lead-free halide double perovskites (HDP), particularly those within the Cs₂B⁺InCl₆ family that show a direct band gap, have recently emerged as promising semiconductors to address key challenges associated with lead-based perovskites, such as toxicity and instability in air and moisture. Their compositional flexibility, structural versatility, and ease of cation transmutation offer considerable potential for bandgap engineering. Here, we present a versatile, multistep, solution-based synthetic strategy for HDP nanocrystals. This method separates the precursor dissolution and reaction stages, providing much greater control over the synthesis. The flexibility of this approach makes it widely generalizable and highly adaptable to high-throughput flow chemistry techniques, including multifluidic platforms and (semi)automated frameworks (e.g., self-driving or automated laboratories). Here, we demonstrate that stable Cs₂(Na,Ag)InCl₆ and Cs₂(Na,K)InCl₆ HDP compositions can be readily obtained through this approach. Through state-of-the-art atomic-scale experimental and theoretical characterization, we provide insights into the evolution of chemical bonding upon Na⁺/Ag⁺ substitution into the Cs₂(Na,Ag)InCl₆ series. Finally, we investigate the limited miscibility of K⁺ within the NaCl₆ sublattice of Cs₂(Na,K)InCl₆, which can be ascribed to the distorted pentagonal bipyramidal coordination adopted by K⁺, as observed in the endmember Cs₂KInCl₆ composition. All together, these fundamental structural findings serve as a basis for the interpretation of the optical properties of the HDP nanocrystals developed in this work. By combining spectral and structural evidence, we investigate the origins of their absorption and emission properties, with general applicability to similar HDP compositions.