Mapping and Controlling Dimensional Polymorphism and Bandgap Bowing in Cs3Bi2-xSbxI9

A3M2X9 halides (A = Rb, Cs; M = Sb, Bi; X = Cl, Br, I) are a major class of lead-free, perovskite-related semiconductors in which polymorphism and dimensionality provide powerful levers for tuning electronic structure. Cs3Bi2-xSbxI9 exemplifies this behavior, exhibiting 0D (α) and 2D (β) polymorphs whose synthetic accessibility varies strongly with composition, temperature, and sealed-ampule conditions. Here, we establish a protocol-dependent accessibility map (i.e., the α/β (or mixed-phase) and CsI outcome as a function of composition, thermal protocol, and the sealed-ampule environment that governs SbI3-mediated transport/concomitant CsI formation) for the Cs3Bi2-xSbxI9 polymorphs. We show that the α-polymorph is accessible across the full Bi/Sb range, whereas the production of predominantly the β-polymorph is restricted to Sb-rich compositions at room temperature (x ≥ 1.42) and is favored under lower-temperature conditions consistent with minimized SbI3 vapor/CsI production. Density functional theory indicates both polymorphs are locally stable, with the α-phase lower in energy but with a diminishing α-β energy gap as Sb content increases, supporting the increased energetic accessibility of the corner-sharing β-phase structures in the Sb-rich regime. Variable-temperature powder X-ray diffraction directly captures high-temperature α ↔ β interconversion in Bi-rich samples. Optoelectronic measurements reveal band gap bowing across the α-series (Eg = 1.89-2.08 eV; b = 0.11 ± 0.02 eV) and a Bi-dependent reduction of Eg in the β-series (1.79-1.95 eV), consistent with calculated band-edge orbital character. The results highlight that vapor-phase halide chemistry and polymorph/phase identity can be decisive hidden variables in composition-property relationships for lead-free perovskite-related semiconductors.