Unraveling Ionic Conductivity Mechanisms in BeF ₂ –NdF ₃ Molten Salts via First-Principles Molecular Dynamics

First-principles molecular dynamics simulations systematically elucidate the influence of atomic structure on ionic conductivity in BeF₂-NdF₃ (FBeNd) molten salt, a key constituent salt in electrochemical pyroprocessing for the molten salt reactor. The increase in ionic conductivity with Nd concentration is explained by multilevel structural analyses encompassing phonon modes, ionic pair structures, network architectures, and electronic characteristics. Phonon dispersion analysis demonstrates that high- and low-frequency vibrational modes are governed by Be and Nd ions, respectively. Detailed structural analyses confirm that enhanced Nd diffusivity correlates with improved Nd-Nd interactions manifested through shortened Nd-Nd distances, distorted Nd-F-Nd angles, emergent edge/face-sharing clusters, and intensified electronic polarization. Conversely, Be-F tetrahedra retain structural integrity with increasing Nd concentrations, and network fragmentation accelerates Be and F diffusion. The dual enhancement effect of ionic self-diffusion coefficients and charge carrier concentration synergistically elevates the bulk ionic conductivity of molten FBeNd. Overall, a composition-structure-property framework spanning macroscale conductivity to atomistic features is established, offering foundational insights for the predictive modeling of fission product accumulation effects and the rational design of separation protocols in pyroprocessing.