THE REDOX CHEMISTRY OF NIOBIUM(V) FLUORO AND OXOFLUORO COMPLEXES IN LIF-NAF-KF MELTS

The electrochemical behavior of niobium(V) fluoro and oxofluoro comple xes in eutectic LiF-NaF-KF (FLINAK) melts at 700 degrees C has been st udied by cyclic voltammetry. The fluoro complexes NbF72-, introduced i nto the melt by the addition of K2NbF7, can be reduced to niobium meta l in two reversible steps involving one and four electrons, respective ly. At 700 degrees C the diffusion constants of the fluoro niobate com plexes 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, r espectively. Titration with equivalent amounts of oxide ions, introduc ed 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 co nversion is more nearly complete than at 700 degrees C, while the pres ence of NbOFn(n-3-) complexes cannot be identified in cyclic voltammog rams obtained at 795 degrees C. It is concluded that the degree of con version decreases with increasing temperature. At Na2O/K2NbF7 molar ra tios equal to three, electroactivity is still observed in the melt, in dicating 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 reducti on of Nb(IV)F-x((x-4)-) ions, and at Na2O/K2NbF7 molar ratios exceedin g two, no deposits are obtained. The reduction of the oxofluoro comple x NbO2F43-, and complexes formed at Na2O/K2NbF7 molar ratios exceeding two always proceed in one step.