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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, 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°C the diffusion constants of the fluoro niobate complexes involved in these reduction steps, i.e., NbF and were determined to be 8.3 X 106 and 3.4 X iO cm2/s, respectively. Titration with equivalent amounts of oxide ions, introduced as Na2O, leads to a conversion of NbF to oxofluoro complexes of the type NbOF3 and NbO2F. At 700°C the conversion of NbF to NbOF3 is not complete, and the degree of conversion is shown to depend stronly on temperature. Thus, at 645°C the conversion is more nearly complete than at 700°C, while the presence of NbOF complexes cannot be identified in cyclic voltammograms obtained at 795°C. It is concluded that the degree of conversion decreases with increasing temperature. At Na20/K2NbF7 molar ratios equal to three, electroactivity is still observed in the melt, indicating the presence of solute secies. The products of reduction of the oxofluoro complexes have not been identified because the reduction of NbOF,°i ons cannot be obtained without simultaneous reduction of Nb(IV)F°4 ions, and at Na2O/K2NbF7 molar ratios exceeding two, no deposits are obtained. The reduction of the oxofluoro complex NbO2F, and complexes formed at Na2O/K2NbF7 molar ratios exceeding two always proceed in one step.

The electrochemical behavior of tantalum in the form of K2TaF7 in an LiF-NaF-KF eutectic melt has been studied by linear voltammetry in the temperature range of 560 to 815~ with and without additions of Na20. An amperometric titration has been performed by measuring the heights of the cathodic and the anodic peaks. It was shown that at a molar ratio Na20/K2TaF7 = 1 the predominating complex in the melt is TaOF~ , whereas with an Na20/K2TaF7 molar ratio of 2 it is TaO2F(= =-I)-, probably in the form of TaO2F~-. Increase in the Na20/K2TaF7 molar ratio in excess of two leads to a decrease of tantalum concentration in the melt, and precipitation of KTaQ occurs. Both the fluoro complex and the monooxefluoro complex were reduced to metal in a single five-electron step. The fluoro complexes, in the temperature range 625 to 815~ with potential scan rates <0.5 V 9 s i, discharge quasi-reversibly, but at potential scan rates >0.5 V 9 s -I they discharge irreversibly. Monooxofluore complexes discharge irreversibly at all temperatures and scan rates studied. The diffusion coefficient of the tantalum fluoro complex depends on the temperature as log D = -2.55 -2044/Twith an activation energy of 39.1 kJ 9 mol -~. For the tantalum monooxofluoro complex the dependence is log D = -2.35 -2293/T with an activation energy of 43.9 kJ 9 mol -I.

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