Unveiling ionic structure in the LiF-NdF3 molten salt system as determined by Raman spectroscopy and quantum chemical calculations

Understanding the structural intricacies of LiF–NdF3 molten salts is of paramount importance for advancing the neodymium electrolysis technology. In this study, the vibrational structure of a LiF–NdF3 molten salt with a LiF/NdF3 molar ratio of 1 was analyzed through in situ Raman spectroscopy, coupled with an in-depth analysis of the ionic structure using molecular simulation techniques. This investigation revealed the presence of various species, including NdF63−, NdF52−, NdF4−, and Nd2F7−, in the molten salt. The primary bands corresponding to these ions were unambiguously identified at wavenumbers of 400, 443, and 491 cm−1, with an intriguing overlapping band at 491 cm−1 corresponding to Nd2F7− and NdF4−. Notably, the results revealed a network of interconversion pathways among these species within the LiF–NdF3 molten salt. Remarkably, Nd2F7− exhibited D3d and D3h group symmetry structures, with a small single-point energy difference of only 0.42 kcal/mol, highlighting their uniform coexistence and stability within the molten salt matrix. Moreover, anodic perfluorocarbon gas was formed by the electrochemical reactions between the F atoms in NdF63−, NdF52−, NdF4−, and Nd2F7− species that were most prone to undergo electrophilic reactions and anodic carbon during the neodymium electrolysis process. By focusing on the complexities of the ionic structure, this study provides insights into the behavior of the LiF–NdF3 molten salt, highlighting its role in the development of neodymium electrolysis technologies.