Organic Cation-Directed Tuning of Structure and Optical Band Gap of One-Dimensional Hybrid Sb(III) Iodides

Abstract Sb(III)-halide perovskite-related structures are environmentally benign alternatives to traditional lead-based perovskites. Here, we report three hybrid Sb(III) iodides: (H2BDA)SbI5, (H2PDA)SbI5, and (H2CDA)SbI5, where H2BDA, H2PDA, and H2CDA represent 1,4-butanediammonium, 1,5-pentanediammonium, and trans-1,4-cyclohexanediammonium cations, respectively. These compounds exhibit one-dimensional zigzag chains of [SbI6]3– octahedra separated by organic cations. Temperature-dependent X-ray diffraction and calorimetry reveal thermally reversible phase transitions for (H2BDA)SbI5 at 368 K upon heating and at 363 K upon cooling, while (H2PDA)SbI5 undergoes a transition at 428 K in the heating scan and at 424 K in the cooling scan. Specifically, (H2BDA)SbI5 undergoes a monoclinic (P21/n) to monoclinic (P21/m) phase change, while (H2PDA)SbI5 changes from orthorhombic (P212121) to orthorhombic (Pnma) at high temperatures. In contrast, (H2CDA)SbI5 does not show any detectable transition within the studied temperature range. The optical band gaps are in the 2.0–2.3 eV range, with (H2CDA)SbI5 exhibiting a significantly wider gap compared to the other two analogs, attributed to pronounced distortion of the Sb–I framework, particularly increased Sb–I–Sb tilting. Additionally, (H2BDA)SbI5 exhibits a broad, Stokes-shifted emission centered at 720 nm and extending to 1200 nm. The broad emission is attributable to self-trapped excitons, making this system a potential light-emitting material.