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 duri ng electrodeposition of tantalum in various TaCl5-alkali chloride melt s at temperatures up to 1100 K. This has been performed by simultaneou s measurements of impedance and Raman spectra together with cyclic vol tammograms. A detailed analysis of electrochemical and spectroscopic r esults yields the following predominant reduction steps during electro deposition in pure alkali chloride melts: Ta(V) -Ta(IV)-Ta(0). The equ ilibrium 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 i ndications 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 electroche mical polarization measurements. These are reported here the first tim e 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 magni tude as ionic conductivities.