Thermodynamic and Electrochemical Studies of Niobium in Molten Fluorides and Chloroaluminates

The oxide chemistry of niobium(V) has been studied in molten LiF-BeF₂ mixtures. The stoichiometry of an oxygen-containing niobium(V) species (NbO₂F) has been established in molten Li₂BeF₄ from the heterogeneous equilibrations of NiNb₂o₆ and NiO with the melt containing NiF₂, the equilibrium quotient for the reaction was determined. The equilibrium quotient for the heterogeneous reaction of Nb₂O₅ and BeO with molten LiF-BeF₂ mixtures (67-33 and 52-48 mole %) was also determined, and the effect of the melt composition on the solubility of Nb₂O₅(c) and on the activity coefficient of NbO₂F(d) have been examined. The free energies of formation of NbO₂F(d) and NiNb₂O₆(c) has been estimated from the experimental results. A Pourbaix diagram for niobium in molten Li₂BeF₄ at 500° and a phases diagram at 600° involving NbO₂F and NiF₂ and the equilibrium oxide phases (Nb₂O₅, NiNb₂O₆, NiO, and BeO) in molten Li₂BeF₄ were constructed.

The electrochemical reduction of Nb(V) in molten LiF-BeF₂-ZrF₄ (65.6-29.4-5.0 mole %) has been studied by linear sweep voltammetry with pyrolytic graphite and platinum electrodes. Potentials were measured with respect to a Ni(II) (saturated)/Ni reference electrode. Nb(V) was found to form NbO₂F. The results at the pyrolytic graphite electrode were reproducible; three reduction steps were observed at low scan rates (< 0.5 V/sec) and one reduction step was observed at high scan rates (> 5 V/sec). Mechanisms are proposed for the reduction of niobium(V). The results with platinum electrode were complicated and irreproducible.

The electrochemical reduction of Nb(V) chloride and Nb(V) oxychloride in molten AlCl₃-NaCl mixtures (63-37 to 50-50 mole %) has been examined by means of chronoamperometry, differential pulse polarography, chronopotentiometry and linear sweep voltammetry using platinum and tungsten electrodes. Potentials were measured with respect to an Al(III) (AlCl₃-NaCl, 63-37 mole %)/Al reference electrode. The effect of the melt composition, temperature and Nb(V) concentration on the reduction steps was studied. The results show that the stability of the niobium species and the electrode reduction mechanisms of Nb(V) chloride and oxychloride are very sensitive to the melt composition and to the temperature. The number of reduction steps, the redox potentials, and the stability of various niobium species at different melt compositions and temperatures are reported. Reduction mechanisms of niobium(V) chloride and oxychloride are proposed.

Some vibrational spectroscopic studies of solid Nb₂O₅ and NiNb₂O₅ were performed; the results indicate that the symmetric stretching frequencies of Nb-O in Nb₂O₅ and NiNb₂O₆ are 992 and 882 cm^-1, respectively.