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 degrees C has been studied by cyclic voltammetry. The fluoro complexes NbF72-, introduced into the melt by the addition of K2NbF7, can be reduced to niobium metal in two reversible steps involving one and four electrons, respectively. At 700 degrees C the diffusion constants of the fluoro niobate complexes involved in these reduction steps, i.e., NbF72- 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 Na2O, leads to a conversion of NbF72- to oxofluoro complexes of the type NbOFn(n-3-) and NbO2F43-. At 700 degrees C the conversion of NbF72- to NbOFn(n-3) is not complete, and the degree of conversion is shown to depend strongly on temperature. Thus, at 645 degrees C the conversion is more nearly complete than at 700 degrees C, while the presence of NbOFn(n-3-) complexes cannot be identified in cyclic voltammograms obtained at 795 degrees C. It is concluded that the degree of conversion decreases with increasing temperature. At Na2O/K2NbF7 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 NbOFn((n-3)-) ions cannot be obtained without simultaneous reduction of Nb(IV)F-x((x-4)-) ions, and at Na2O/K2NbF7 molar ratios exceeding two, no deposits are obtained. The reduction of the oxofluoro complex NbO2F43-, and complexes formed at Na2O/K2NbF7 molar ratios exceeding two always proceed in one step.