TECHNICAL PAPERS ELECTROCHEMICAL SCIENCE AND TECHNOLOGY Electrochemical Studies of Tantalum in Fluorochloroaluminate Melts at 200-450~

The electrochemistry of tantalum(V) species in sodium fluorochloroaluminate melts (10 mole percent NaF) has been investigated in the temperature range of 200 to 450 ~ using cyclic, normal pulse, and square wave oltammetries, exhaus-tive electrolysis, Raman and electronic spectroscopies, and x-ray diffraction methods. The electrochemical behavior of tantalum(V) is strongly dependent on temperature. Three main reduction waves are observed at a temperature of 300 ~ or higher. The first and second reduction waves merge into one wave at temperatures below 300 ~ The first reduction wave is associated with the reduction of tantalum(V) to tantalum(IV) Species followed by adimerization reaction which occurs very slowly at lower temperatures. The second reduction wave is believed to be the reduction of the tantalum(IV) dimer, Ta ~ +, to a tantalum(III) species (probably Ta~+). The tantalum(III) species decomposes resulting in the formation of the cluster, Ta6CI~. The last reduction wave is assigned to the reduction of the trivalent tantalum species to a divalent tantalum species, which is highly unstable and decomposes toform the tantalum cluster, Ta~CI~L and metallic tantalum. The clusters are slowly reduced to metallic tantalum. The electrochemistry of tantalum is complicated by the existence of various compounds with different oxidation states, such as Ta 5 § Ta 4., Ta 3 § Ta ~ ~ § Ta ~ 4 § Ta ~ 1-4 in A1C13