Electrochemical studies of the molten system K2NbF7-Na2O-Nb-(LiF-NaF-KF)(eut) at 700 degrees C

Various voltammetric methods have been used to study FLINAK (LiF-NaF-KF eutectic melt 46.5-11.5-42 mol %) melts containing about 1 mol % niobium-fluoro and -oxofluoro complexes with Nh in oxidation states (V) and (IV) at 700 degrees C and varying amounts of Na2O in the range 0 <n(O)(0)/n(Nb)(0) <7. A slow, spontaneous reduction of Nb(V)-fluoru complexes to Nb(IV)-fluoro, complexes was observed in the absence of Nb metal. With a stoichiometric amount of Nb metal, Nb(V) is reduced rapidly to Nb(IV) according to the reaction 4NbF(7)(2-) + Nb reversible arrow 5NbF(x)((x-4)-). The solubilities of Nb2O5, and KNbO3 in FLINAK were measured and found to be 1.8 and 0.13 mol %, respectively The following reactions probably occur when increasing amounts of oxide are added to a melt originally containing NbFx(x-4)-: NbFx(x-4)- + O-2(-) reversible arrow Nb(IV)-mono-oxo-fluoride + fluoride, 5Nb(IV)-mono-oxo-fluoride + O-2(-) reversible arrow 4NbO(2)F(4)(3-) + Nb, and NbO2F43- + O-2(-) + Alk(+) reversible arrow AlkNbO(3)(s) + 4F(-) (Alk(+) = Li+, Na+, or K+). At oxide to niobium molar ratios higher than three, AlkNbO(3)(s) redissolves, and at n(O)(0)/n(Nb)(0) > 4, all Nb and O added are dissolved. Strong indications of the coexistence of the oxygen rich Nb(V)OF complexes and O-2(-) ions in FLINAK at n(O)(0)/n(Nb)(0) > 4 have been found. An equilibrium/sampling/analysis technique was also used to study this system without Nb metal added. The results mainly agree with the results of the voltammetric studies. However. no indications of spontaneous reduction of Nb(V) to Nb(IV) were observed, even after 24 h. (C) 2000 The Electrochemical Society. S0013-3651(99)12-112-8.All rights reserved.