Thermodynamic and Electrochemical Characterization of Nd* (III) Ion Diffusion in (LiF-CaF2)-Nd2O3 Molten Salts.

Abstract

Data on the diffusion and migration characteristics of rare earth metal ions in fluoride molten salt systems are crucial for optimizing the electrolytic preparation of rare earth metals and alloys. This study investigated the solubility, conductivity, and density of the (LiF-CaF₂)_eut. system saturated with Nd₂O₃ using the isothermal saturation method, conductivity cell constant variation, and the Archimedes method, respectively. Employing the Hittorf method's principles, a three-compartment electrolyzer was designed to determine the mobility number of dissolved Nd* (III) ions in the saturated (LiF-CaF₂)_eut.-Nd₂O₃ system. The radial distribution function was computed via ab initio molecular dynamics, and the self-diffusion coefficient of ions in the system was analyzed. Utilizing the Nernst-Einstein equation, the diffusion coefficient of Nd* (III) ions was calculated. The solubility, conductivity, and density of the saturated (LiF-CaF₂)_eut.-Nd₂O₃ system exhibit linear variation within 1173-1473 K. The mobility number of solvated Nd* (III) ions increases linearly with temperature, displaying nonlinear variation with potential within 3.5-4.5 V, and gradually decreases after reaching a maximum of 4.0-4.25 V. The radial distribution function reveals the highest diffusion and mobility barriers for Nd* (III) ions, with solvated O* (II) ions presenting the most significant hindrance. The Nd* (III) ion diffusion coefficients linearly increase with temperature (1123-1373 K) under specific potential conditions (3.5-4.5 V) but exhibit nonlinear changes with potential (3.5-4.5 V) under fixed temperature conditions (1123-1373 K), then decrease after peaking within 4.0-4.5 V. The diffusion coefficients of Nd* (III) ions are sensitive to potential changes.