Dual-Site Anion Modulation of Charge Transport in Kagome Francisites
Cong Li1, Yang Yang1, Xiaoliang Zhang2
1Key Laboratory of Intelligent Optoelectronic Devices and Chips of Jiangsu Higher Education Institutions and Advanced Technology Research Institute of Taihu Photon Center, School of Physical Science and Technology, Suzhou University of Science and Technology, Suzhou, 215009, China.
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.
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