Reduction products of pentavalent Niobium and Tantalum in fused chloride solvents

G. Voyiatzis, E.A. Pavlatou, G.N. Papatheodorou, M. Bachtler, W. Freyland

The valence states and species formed by metal or electrolytic reduction of TaCl5 and NbCl5 in molten alkali chloride solvents have been determined by Raman spectroscopic measurements. In the molten solvents CsCl(l), NaCl/CsCl(eut) and LiCl/KCl(eut) the refractory metal pentachlorides MCl5 (M=Ta, Nb) were systematically reduced by adding the corresponding metal M and a series of metals L=Au, Ag, Cu, Pb, Cd, Zn and Al. Raman spectroscopic measurements of these solutions indicated that the predominant species present were the MCl6- and MCl62- octahedral having the refractory metal in the valence states five and four respectively. Equilibrium between the MCl6- and MCl62- species appear to exist in the melt solvents and no other lower valence states could be detected. In situ Raman spectroelectrochemical measurements on amorphous carbon electrodes in LiCl/KCl(eut) containing almost 5mol% TaCl5 indicated that the tetravalent TaCl6-2 species is formed on or in the vicinity of the cathode. Another tantalum chloride species with valence state lower than four was also detected from the electrode spectra.  

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