ZnI₂-Mediated Synthesis of High-Purity Rare Earth Iodides with Systematic UV-Vis-NIR Absorption Signatures

Anhydrous, donor-free rare earth iodides are key precursors for rare earth chemistry and functional materials, but their scalable synthesis is challenging because these salts are extremely sensitive to air and moisture. In addition, most established routes rely on toxic, corrosive, or otherwise nonbenign reagents. Here, we report a ZnI₂-mediated solid-state route to rare earth iodides in which ZnI₂ acts simultaneously as the iodide source and oxidant, enabling a simple and scalable preparation. This approach leverages the good safety profile and low melting point of ZnI₂ and the low boiling point of Zn as a removable byproduct, even though such zinc halides have rarely been used as sacrificial halogenating agents for high-purity inorganic halides. The workflow comprises (i) reaction of ZnI₂ with rare earth metals, (ii) removal of Zn/ZnI₂ byproducts under vacuum, and (iii) two-zone vacuum sublimation to afford highly crystalline, high-purity rare earth iodides. Using this protocol, we obtained 13 rare earth triiodides and the divalent diiodides of Eu and Yb. The high purity of these materials enables systematic UV-vis-NIR diffuse-reflectance spectroscopy under unified synthetic and measurement conditions, allowing comparative analysis of 4f-4f, 4f-5d, and iodide-to-metal charge-transfer absorptions across both diiodides and triiodides.