Polyhalide Formation in Metal-Free Perovskites: An X-ray Absorption Spectroscopy Study

Metal-free perovskites (MFPs) are generally regarded as wide-bandgap insulators, yet we recently observed visible-light absorption in DABCO–NH₄X₃ powders after short mechanosynthesis. The microscopic origin of this unexpected activity is unknown. For iodide-based perovskites, solvent extraction and UV–Vis measurements strongly suggest the presence of I₃⁻, though we cannot exclude other Iₙ⁻ species. X-ray absorption spectroscopy (XAS) provides a unique and direct probe of local bonding environments and can distinguish between monohalide anions (X⁻) and polyhalide species (Xn⁻) through their characteristic edge shifts and fine structure. We propose to carry out variable-temperature XAS at the halide K-edges (Br, I) to test whether mechanochemistry induces polyhalide formation in MFPs and to establish how such species correlate with the onset of visible absorption.