Structural, electric and dynamic properties of (pyrrolidinium)3[Bi2I9] and (pyrrolidinium)3[Sb2I9]; new lead-free organic-inorganic hybrids with narrow band gaps

We present a detailed physicochemical characterization of newly designed pyrrolidinium templated bismuth(III) and antimony(III) iodides. Both compounds consist of a zero-dimensional anionic substructure, which is composed of face-sharing octahedra. The pyrrolidinium cations act as spacers and compensate for the charge. By utilizing diffraction experiments on single-crystals and powders, UV-vis, dielectric and NMR spectroscopy, and DFT calculations, we have developed PT diagrams and identified molecular order-disorder processes. Furthermore, we show that both compounds have a semiconducting character and exhibit a high dielectric constant. This combination, along with their exceptional chemical stability, makes these new lead-free organic-inorganic hybrids highly promising for optoelectronic applications.