Exploring compositional versatility of perovskite-like Cs3(Bi,Sb)2X9 (X = Cl, Br, I) compounds by high-throughput experimentation

Abstract

A high-throughput compositional screening of perovskite-like Cs₃M₂X₉ double salts (M = Bi and Sb and X = Cl, Br, and I) allows independent variation of the M and X components, yielding one hundred single-phase products within a general synthetic approach that combines engineered precipitation of chloride and bromide precursors and their anion exchange conversion into more complex halide derivatives. The X variation at a fixed M = Bi³⁺ yields various single-phase Cs₃Bi₂X₉ compounds with X = Cl, Cl + Br, Br, Br + I, and Cl + Br + I. The anion exchange in chlorides with Br + I combinations produces stable Cs₃Bi₂X₉ compounds with all three halides simultaneously present in the lattice, and Cl, Br, and I contents varied in the ranges of ca. 40–90%, 10–60%, and 30–90%, respectively. The presence of bromide, even as a residue, enables the co-existence of Cl and I, and dictates the trigonal symmetry, in contrast to the hexagonal symmetry typical for Cs₃M₂I₉. The compounds with X = Cl + Br + I show band gap variations in the range of 2.0–2.5 eV and linear dependencies on the iodide content and lattice parameters. The simultaneous variation of the X and M sites yields single-phase Cs₃(Bi,Sb)₂X₉ solid-solution compounds with tailorable X and a Bi/Sb ratio varied from 0 to 1.0. All Bi/Sb families reveal a band bowing effect, with the band gaps of mixed Bi/Sb compounds being lower than those of Bi- and Sb-only counterparts. The bowing parameter depends on the X subsystem, decreasing from 0.80 eV for Cl to 0.60 eV for Cl + Br and 0.40–0.45 eV for Br and Br + I, indicating that chemical variations in the mixed Bi/Sb lattices, rather than local disorders or lattice strains, govern the band-bowing behavior.