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 NbF 7 2- , introduced into the melt by the addition of K 2 NbF 7 , 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., NbF 7 2- and Nb(IV)F x (x-4)- were determined to be 8.3 x 10 -6 and 3.4 x 10 -5 cm 2 /s, respectively. Titration with equivalent amounts of oxide ions, introduced as Na 2 O, leads to a conversion of NbF 7 2- to oxofluoro complexes of the type NbOF n (n-3)- and NbO 2 F 4 3- . At 700°C the conversion of NbF 7 2- to NbOF n (n-3)- 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 NbOF n (n-3)- complexes cannot be identified in cyclic voltammograms obtained at 795°C. It is concluded that the degree of conversion decreases with increasing temperature. At Na 2 O/K 2 NbF 7 molar ratios equal to three, electroactivity is still observed in the melt, indicating the presence of solute sqecies. The products of reduction of the oxofluoro complexes have not been identified because the reduction of NbOF n (n-3)- ions cannot be obtained without simultaneous reduction of Nb(IV)F x (x-4)- ions, and at Na 2 O/K 2 NbF 7 molar ratios exceeding two, no deposits are obtained. The reduction of the oxofluoro complex NbO 2 F 4 3- , and complexes formed at Na 2 O/K 2 NbF 7 molar ratios exceeding two always proceed in one step