ELECTROCHEMICAL AND SIMULTANEOUS SPECTROSCOPIC STUDY OF REDUCTION-MECHANISM AND ELECTRONIC CONDUCTION DURING ELECTRODEPOSITION OF TANTALUM IN MOLTEN ALKALI CHLORIDES

We have studied the reduction mechanism and electronic conduction during electrodeposition of tantalum in various TaCl5-alkali chloride melts at temperatures up to 1100 K. This has been performed by simultaneous measurements of impedance and Raman spectra together with cyclic voltammograms. A detailed analysis of electrochemical and spectroscopic results yields the following predominant reduction steps during electrodeposition in pure alkali chloride melts: Ta(V) -Ta(IV)-Ta(0). The equilibrium potentials of the corresponding redox reactions are separated by 40 mV which is clearly resolved in impedance measurements. In LiCl/KCl-eutectic melts a three step mechanism is found with Ta(III) being the intermediate species. In evaluating the impedance data we found indications for electronic contributions which are most prominent with added oxide impurities to the molten electrolyte. Direct evidence for an appreciable electronic conductivity has been obtained by electrochemical polarization measurements. These are reported here the first time for fused KCl-TaCl5/Ta at 1050 K, where electronic conductivities up to 0.5 Ohm(-1) cm(-1) have been measured, i.e. of same order of magnitude as ionic conductivities.