Competitive Coordination of Chloride and Fluoride Anions Towards Trivalent Lanthanide Cations (La3+ and Nd3+) in Molten Salts

Molten salt electrolysis is a vital technique to produce high-purity lanthanide metals and alloys. However, the coordination environments of lanthanides in molten salts, which heavily affect the related redox potential and electrochemical properties, have not been well elucidated. Here, the competitive coordination of chloride and fluoride anions towards lanthanide cations (La3+ and Nd3+) is explored in molten LiCl-KCl-LiF-LnCl(3) salts using electrochemical, spectroscopic, and computational approaches. Electrochemical analyses show that significant negative shifts in the reduction potential of Ln(3+) occur when F- concentration increases, indicating that the F- anions interact with Ln(3+) via substituting the coordinated Cl- anions, and confirm [LnCl(x)F(y)](3-x-y) (y(max)=3) complexes are prevailing in molten salts. Spectroscopic and computational results on solution structures further reveal the competition between Cl- and F- anions, which leads to the formation of four distinct Ln(III) species: [LnCl(6)](3-), [LnCl(5)F](3-), [LnCl(4)F(2)](3-) and [LnCl(4)F(3)](4-). Among them, the seven-coordinated [LnCl(4)F(3)](4-) complex possesses a low-symmetry structure evidenced by the pattern change of Raman spectra. After comparing the polarizing power (Z/r) among different metal cations, it was concluded that Ln-F interaction is weaker than that between transition metal and F- ions.

Automated summary

The research explores the competitive coordination of chloride and fluoride anions towards lanthanide cations in molten salts, showing that the addition of fluoride ions significantly shifts the reduction potential of Ln(3+). The study also reveals the formation of predominant [LnCl(x)F(y)](3-x-y) complexes in molten salts and the weaker Ln-F interaction compared to transition metal and F- ions.