EFFECT OF OXYGEN ON THE COMPLEXING AND ELECTROCHEMICAL PROCESSES IN THE NACL-KCL-K2TAF7 MELT

The influence of oxygen on the electrochemical behavior of tantalum du ring titration of the NaCl-KCl-K2TaF7 melt with sodium oxide is studie d by the linear voltammetry method. As oxygen replaces the fluorine in the ligand shell of the TaClF73- complex, the peak corresponding to i t diminishes, and a peak of discharge of tantalum hydroxyhalide appear s and grows in the more negative region of potentials. At the molar ra tio O/Ta = 1, tantalum reduces from TaOClF53-, which is the only compl ex observed in the melt. The tantalum reduction at polarization rates v greater than or equal to 1 V s(-1) occurs irreversibly. The diffusio n coefficient for TaOClF53- at 750 degrees C is equal to about 1.1 x 1 0(-5) cm(2) s(-1). At v < 1 V s(-1), the reduction process is complica ted by the formation of tantalum hydroxycarbides at the glassy-carbon electrode and intermetallic Pt-Ta compounds at the platinum electrode. No additional peaks appear in the voltammograms upon a further titrat ion of the melt with sodium oxide (1 < O/Ta < 2.4), but the height of the current peak corresponding to the reduction of the TaOClF53- compl exes decreases because of the formation of the K0.4TaO2.4F0.6 compound which is insoluble in the melt. An increase in the O/Ta ratio above 2 .4 leads to the tantalum concentration in the melt dropping to zero an d to depositing on the crucible bottom of KTaO3 along with K0.4TaO2.4F 0.6.