Electrodeposition processes of tantalum(V) species in molten fluorides containing oxide ions

The influence of the molar ratio O-Ta noted R-o on the electrodeposition of tantalum in molten fluorides at 800 degreesC is examined. After additions of both K2TaF7 and Na2O, we observed: (i) tantalum deposit is obtained only by reduction of TaF72- (ii) the oxide ions react with TaF72- to give oxifluoride species, identified first as TaOF52- and for higher oxide content, as TaO2F43-; (iii) the electroreduction of TaOF52- leads to the formation of an unstable bivalent tantalum species (TaO); (iv) the conversion of TaF72- to oxifluoride is complete when R-o is 1. Titration of Ta by ICP and oxygen by a carbothermal-reduction technique show that for R-o > 1, insoluble compounds Na3TaO4 and NaTaO3 are formed. For R-o > 3, these species seem to be solubilised. Electrodeposition runs on copper cathodes show that pure and coherent Ta coatings are obtained for a lower oxide content (R-o < 0.5). For higher R-o, the faradic efficiencies are low. The coatings are powdery and include the presence of Na3TaO4, NaTaO3 and traces of Ta metal, arising from the decomposition of TaO. (C) 2002 Published by Elsevier Science Ltd.