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Niobium Complexes in Fluoride Melts

V. Van, J. Madejová, A. Silný, and V. Daněk

Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-842 36 Bratislava

 

Abstract: Complex formation and redox chemistry of the niobium complex in fluoride melts has been studied by cyclic voltammetry and chronopotentiometry. The X-ray powder diffraction analysis and IR spectroscopy was used to clarify the complex formation in the high oxidation states of niobium metal. Two-step reversible diffusion-controlled mechanism in the melt at 750°C Nb(V)→ Nb(IV) → Nb(0) was confirmed. In the presence of oxide ions in fluoride melts niobiumoxyfluoro complexes are formed. At low oxide concentration mono-oxyfluoro complexes have the form of [Nb(V)OF5]2-. With the nO: nNb(v) ratio higher than 0.7 the complex changes to the [Nb(V)OF6]3- one. The ligand displacement of the niobium fluoro complexes to the mono-oxyfluoro complexes is not complete and the conversion degree was found to be 0.7. Free oxide ions cause di-oxyfluoro complexes formation that both oxyfluoro complexes coexist despite the low oxide concentration in the melt. The diffusion coefficient of oxyfluoro complexes in fluoride melts depends on the nO: nNb(v) ratio in the melts. With the nO: nNb(v) ratio up to 0.7 there is no change in the diffusion coefficient of the monooxyfluoro complex. The coefficient strongly increases in the ratio interval 0.7-0.9 and then again decreases.

Full paper in Portable Document Format: 543a137.pdf

 

Chemical Papers 54 (3) 137–143 (2000)

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