Dual-Site Anion Modulation of Charge Transport in Kagome Francisites

Abstract Mixed-anion chemistry provides a versatile route for controlling bonding, lattice geometry, and electronic localization in complex oxides, yet its role in kagome-lattice francisites remains insufficiently understood. Here, we report a systematic dual-site anion substitution study of francisite-type oxides, Cu3BiSexTe2–xO8ClyBrzI1–y–z, in which both the chalcogen site (Se/Te) and halogen site (Cl, Br, I) are chemically varied. Synchrotron powder X-ray diffraction and Rietveld refinement show that the parent francisite framework is retained across all compositions, while the lattice responds anisotropically to halogen substitution and more locally to Se/Te substitution. These crystallographic changes are accompanied by pronounced variations in particle morphology, optical absorption, and macroscopic resistance. In particular, pure-iodide composition exhibits the lowest resistance, whereas chloride-containing and Te-substituted samples are substantially more resistive. DFT calculations reveal that this transport contrast arises from the cooperative regulation of carrier activation, charge localization, and band-edge dispersion by the two anion sublattices. Iodide incorporation promotes a less localized and more dispersive band-edge electronic structure, whereas Cl enhances local charge accumulation and Te perturbs the chalcogen-derived valence-band network. This work establishes a chemically intuitive structure–property relationship for anion-site control of charge localization in francisite oxides and highlights mixed-anion substitution as a design strategy for tuning transport in kagome oxide frameworks.