Structure of Molten MX–NdX3 (M = Na, K, Rb, Cs; X = F, Cl) Salts: Ab Initio Study

Abstract—The structures of model analogs of salt melts containing neodymium complexes are subjected to ab initio studies. The importance of this work is mainly determined by the need to develop methods and technologies for recycling electronic and magnetic materials, which are a secondary source of rare earth metals. Quantum-chemical calculations, in turn, are a powerful tool for studying the structural features of model analogs of real high-temperature salt melts. The calculations are performed by the Hartree–Fock and density functional theory methods using the quantum-chemical Firefly 8.20 software package. An approach is proposed to calculate the interaction energies in a three-component model system consisting of a neodymium complex, an outer-sphere (OS) cation shell, and the remaining part of the cluster. The energies of interaction of the neodymium complex with the system fragments are determined. The influence of the number of OS cations on the calculated interaction energies has been studied, and the compositions of the most stable particles consisting of neodymium complex + OS shell have been determined. The data obtained using quantum-chemical calculations are compared with the results available in the world scientific literature, including the results of direct spectroscopic studies of molten salts containing neodymium complexes. The calculated interatomic distances Nd–X (X = F, Cl) are shown to agree well with the available experimental data. Raman spectra are calculated for 18MCl + M₃NdCl₆ (M = Na, K, Rb, Cs) model systems, and the calculated and experimental frequencies of the most intense spectral line are shown to be in good agreement. The results obtained allow us to assert that the model systems studied in this work can be used as the minimum possible ones for studying the structures of molten salts using quantum-chemical methods.