The electrochemical behavior of tantalum in the form of K2TaF7 in an LiF-NaF-KF eutectic melt has been studied by linear voltammetry in the temperature range of 560 to 815-degrees-C with and without additions of Na2O. An amperometric titration has been performed by measuring the heights of the cathodic and the anodic peaks. It was shown that at a molar ratio Na2O/K2TaF7 = 1 the predominating complex in the melt is TaOF5(2-), whereas with an Na2O/K2TaF7 molar ratio of 2 it is TaO2F(x)(x-1-), probably in the form of TaO2F4(3-). Increase in the Na2O/K2TaF7 molar ratio in excess of two leads to a decrease of tantalum concentration in the melt, and precipitation of KTaO3 occurs. Both the fluoro complex and the monooxofluoro complex were reduced to metal in a single five-electron step. The fluoro complexes, in the temperature range 625 to 815-degrees-C with potential scan rates 0.5 V . s-1 they discharge irreversibly. Monooxofluoro complexes discharge irreversibly at all temperatures and scan rates studied. The diffusion coefficient of the tantalum fluoro complex depends on the temperature as log D = -2.55 - 2044/T with an activation energy of 39.1 kJ . mol-1. For the tantalum monooxofluoro complex the dependence is log D = -2.35 - 2293/T with an activation energy of 43.9 kJ . mol-1.
Electrochemical Society. Journal, 1994, Vol 141, Issue 11, p. 2982-2988