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.