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The mechanism of the electroreduction of niobium chlorides dissolved in fused was determined by transient electrochemical techniques at temperatures ranging from 670 to 870°C. At temperatures <750°C it is shown that the stable oxidation states of niobium ions are Nb(V), Nb(IV), and Nb(III); reduction of Nb(III) gives rise to metallic niobium. At higher temperatures, Nb(V) tends to decompose into Nb(IV) with chlorine evolution. Introducing fluoride ions stabilizes the higher oxidation states. For a molar ratio F/Nb, , Nb(V) is stable over the whole temperature range and the direct reduction is obtained as the Nb(III) species is no longer stable. The standard potentials of the redox reactions are calculated for various concentrations of fluoride ions. Presence of fluoride ions has a beneficial effect on the quality of the niobium deposit, a ratio is enough for obtaining coherent deposits.
Lantelme et al. (Thu,) studied this question.