Mechanism and kinetics of niobium ion reduction in LiF-NaF-K2NbF7 melts

The mechanism and kinetics of niobium ion reduction in LiF-NaF-K2NbF7 melt were studied by cyclic voltammetry and chronopotentiometry. Two-step revers ible diffusion-controlled mechanism in the melt at 750 degrees C was found to be Nb(V) --> Nb(IV) --> Nb(0). The effect of added oxide ions on the nio bium ion reduction process was discussed on recorded chronopotentiograms. T he formation of niobium mono-oxyfluoro complex in the melt at 750 degrees C with an m(O):n(Nb(V)) ratio up to unity was observed. The ligand displacem ent reaction [Nb(V)F-7](2-) + O2- = [Nb(V)OF5](2-) + 2F(-) is not complete at 750 degrees C and the conversion degree was found to be 0.7. The diffusi on coefficient of [NbF7](2-) at 750 degrees C ion was found to be D-Nb(V) = 3.2 X 10(-5) cm(2) s(-1). The diffusion coefficient of mono-oxyfluoro comp lexes in fluoride melts depends an the n(O):n(Nb(V)) ratio in the melts; fo r ratios up to 0.7 there is no change in the diffusion coefficient of the m ono-oxyfluoro complexes. The coefficient strongly increases in the ratio in tenval 0.7-0.9 and then again decreases. (C) 1999 Elsevier Science S.A. All rights reserved.