Atomic-Scale Insights into Interfacial Dynamics between LiF-BeF ₂ –SmF ₃ Molten Salt and Solid Aluminum

Local structures and dynamic behavior of molten LiF-BeF 2 (FLiBe) salt and solid metallic aluminum (Al) are investigated using ab initio molecular dynamics (AIMD) simulations at 823 K, aiming to elucidate the separation mechanism of the soluble fission product SmF 3 . Results suggest that the atomic-scale FLiBeSm-Al interface presents an opportunity to manipulate charge and drive electrochemical transformations. Charge density analysis indicates that electron transfer from Al to Sm occurs via bridged outer-sphere ions, with Al atoms carrying a slight positive charge. Among all cations, Sm exhibits the largest charge transfer (1.04 ± 0.09 e ) and a notable charge difference between the bulk and interfacial region, suggesting a pronounced valence reduction from Sm 3+ to Sm 2+ at the interface under the applied electric field (EF). Furthermore, structural analysis reveals an interaction between Sm and Al, as evidenced by its peak intensity and the largest coordination number among all interfacial ion-Al pairs. Based on the first valley distances of Al-related and cation-F pairs, the compact and diffusion layer thicknesses are approximately 2.5 and 5.0 Å, respectively. Finally, analysis of ionic self-diffusion coefficient in different regions and directions shows that the diffusion near the interface and perpendicular to the EF direction is enhanced. Overall, this work provides some insights into the interface chemistry of a two-phase system and helps establish conceptual links between atomic-scale phenomena and macroscopic manifestations of surface reactivity.