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

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 degrees C-550 degrees 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.