Lead-Free Double Cs ₂ Ag(Bi,Sb)(Br,I) ₆ Perovskites: Going below 1.8 eV Bandgap by Anion Exchange and Solid-State Reactions

Anion exchange of microcrystalline Cs2AgBiBr6 double perovskite (CAB-B) with NaI under ambient conditions yielded mixtures of Cs3Bi2(Br,I)9 (CB-(B)I) and CsAg2I3 (CA-I) double salts, which were transformed by annealing at 250–300 °C into a tetragonal Cs2AgBi(Br,I)6 double perovskite (CAB-(B)I) with ca. 80 mol % iodide. The thermally activated solid-state reaction between CB-(B)I and CA-I was confirmed by annealing mechanically mixed CB-(B)I and CA-I, which yielded CAB-(B)I perovskites. Optimization of the anion exchange and the solid-state reaction (AE/T) resulted in phase-pure CAB-(B)I perovskite with ca. 90 mol% iodide and a band gap slightly below 1.9 eV. The general applicability of the proposed approach was demonstrated in a series of AE/T-driven transformations of more complex precursors, including the conversion of Cs2AgBixSb1–xBr6 into tetragonal Cs2AgBixSb1–x(Br,I)6 perovskites with variable x, the highest iodide content of ca. 90%, and the lowest band gap of 1.78 eV observed at a Bi/Sb ratio of 1:1, as well as solid-state reactions between ternary mixtures of CB-(B)I, CA-I, and Cs3Bi2Br9 double salts, yielding tetragonal Cs2AgBi(BryI1–y) perovskites with linear compositional variations of the lattice parameters and band gaps over a broad range of y = 0.08–0.76. The reported two-stage AE/T approach is highlighted as a general, flexible, and sustainable pathway for the combinatorial synthesis of stable tetragonal double perovskites with variable compositions and levels of complexity.