Reaction pathways for the formation of complex fluorides revealed by in situ synchrotron X-ray diffraction

Abstract

This study investigated the reaction between LiF, Mo metal, and CuF₂ as a solid-state fluorine source to form a complex fluoride, tetragonal trirutile (TR)-type Li₂MoF₆, in an Ni metal tube by in situ synchrotron X-ray diffraction (SXRD) experiments. As a result, two formation pathways of TR-type Li₂MoF₆ were identified: one involving a reaction among intermediate phases, i.e., MoF₃, and an unknown Mo-containing fluoride, and the starting materials, i.e., LiF, Mo, and CuF₂, the other involving a direct reaction between LiF, Mo, and CuF₂. The combination of in situ SXRD with differential scanning calorimetry (DSC) revealed that the direct reaction was facilitated by the presence of the liquid phase of LiF-CuF₂. Further, the first-order phase transition of Li₂MoF₆ with a great volume change of 21% was found to occur in the vicinity of 520 °C–550 °C reversibly. Finally, this study demonstrated that an in situ SXRD experiment supported by DSC measurement is a powerful tool for revealing the reaction routes and identifying new phases occurring in sample-loaded metal tubes.