The Redox Chemistry of Niobium(V) Fluoro and Oxofluoro Complexes in LiF‐NaF‐KF Melts

The electrochemical behavior of niobium(V) fluoro and oxofluoro complexes in eutectic LiF‐NaF‐KF (FLINAK) melts at 700°C has been studied by cyclic voltammetry. The fluoro complexes , introduced into the melt by the addition of , can be reduced to niobium metal in two reversible steps involving one and four electrons, respectively. At 700°C the diffusion constants of the fluoro niobate complexes involved in these reduction steps, i.e. , and , were determined to be and , respectively. Titration with equivalent amounts of oxide ions, introduced as , leads to a conversion of to oxofluoro complexes of the type and . At 700°C the conversion of to is not complete, and the degree of conversion is shown to depend strongly on temperature. Thus, at 645°C the conversion is more nearly complete than at 700°C, while the presence of complexes cannot be identified in cyclic voltammograms obtained at 795°C. It is concluded that the degree of conversion decreases with increasing temperature. At molar ratios equal to three, electroactivity is still observed in the melt, indicating the presence of solute species. The products of reduction of the oxofluoro complexes have not been identified because the reduction of ions cannot be obtained without simultaneous reduction of ions, and at molar ratios exceeding two, no deposits are obtained. The reduction of the oxofluoro complex , and complexes formed at molar ratios exceeding two always proceed in one step.