ELECTROCHEMICAL STUDIES OF TANTALUM IN FLUOROCHLOROALUMINATE MELTS AT 200-450-DEGREES-C

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-degrees-C using cyclic, normal pulse, and square wave voltammetries, exhaustive 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-degrees-C or higher. The first and second reduction waves merge into one wave at temperatures below 300-degrees-C The first reduction wave is associated with the reduction of tantalum(V) to tantalum(IV) species followed by a dimerization reaction which occurs very slowly at lower temperatures. The second reduction wave is believed to be the reduction of the tantalum(IV) dimer, Ta2(8+), to a tantalum(III) species (probably Ta2(6+)). The tantalum(III) species decomposes resulting in the formation of the cluster, Ta6Cl123+. 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 to form the tantalum cluster, Ta6Cl122+, and metallic tantalum. The clusters are slowly reduced to metallic tantalum.