Electrochemical Behavior of Si(IV) and Formation of Si–Nd Intermetallics in NaF–BeF2 Molten Salt

The electrochemical reduction behavior of Si(IV) and the codeposition of Si–Nd intermetallics were systematically investigated in a NaF–BeF2 (FNaBe, 57–43 mol %)-based molten salt at 873 K. Voltammetry and potentiostatic electrodeposition studies reveal a two-step reduction pathway for Si(IV): Si(IV) → Si(II) → Si(0), accompanied by underpotential deposition of MoSi2 on the Mo substrate. Raman spectroscopic analysis confirms the presence of Si(II) species, generated via comproportionation between Si(IV) and Si(0), providing further evidence for the proposed reduction mechanism. Thermodynamic evaluation and experimental results demonstrate that Si exhibits a strong polarizing effect on the reduction of Nd(III), promoting the formation of NdSi2 intermetallics. The equilibrium potential of the NdSi2/(Si, NdF3) couple was measured in the FNaBe–Na2SiF6–NdF3 melt as a function of the Nd(III) concentration, yielding a consistent formal standard potential of 0.460 V vs BeF2/Be (RSD = 0.0072). The relatively high formation potential of NdSi2 and its high neodymium content make Si an efficient medium for extracting Nd from FNaBe molten salts. This was validated by applying a cathodic potential of 0.10 V to a Mo electrode, which resulted in over 99% removal of Nd(III) from the FNaBe–NdF3–Na2SiF6 melt via Nd-Si codeposition. Given that thermodynamic evaluations of LnSi2 (Ln = Ce, Pr, Nd, Sm, Eu, Gd) show similar standard formation potentials, the proposed method may be extended to the extraction of other lanthanides in future studies.