Chemical reactions and cathodic products in fluoride melts containing tantalum oxocompounds

  L.P.Polyakova, E.G.Polyakov, P.Taxil, A.V.Arakcheeva, V.V.Grinevich.

 

1,2  Institute of Chemistry KSC RAS Apatity, 184200, Russia

3    University P.Sabatier, Toulouse, 31062,France

4,5  A.A.Baikov Institute of Metallurgy and Material Science RAS, Moscow

 

  Fluoride melts containing tantalum are used for electrolytical deposition of surface layers of this metal. As it was shown earlier, the oxygen impurity causes the  formation of tantalum oxofluorides hindering the production of high-quality coatings. As it has been established recently, such electrolytes are promising for the electrochemical synthesis of tantalum oxocompounds, including bronze, where the metal oxidation state is lower than five,The purpose of this work is studying the chemical reactions and cathodic products during the electrolysis of FLINAK melt containingtantalum oxofluoride complexes at different O/Ta molar ratios.

  It was shown that tantalum coating on steel and copper substrates can be produce at the ratio 0<O/Ta<1 in the electrolyte. In this case the melt contains tantalum monoxofluoro complexes alongside with fluoro ones. According to XRD, IR spectroscopy and optical crystallography, at O/Ta=1 samples of the solidified melts contain K3TaOF6.

  When the oxide ion content in the melt is increased, i.e. at 1<O/Ta<2, on the voltammetric curves two peaks of tantalum reduction from, apparently, TaOF5(2-) and TaO2F4(3-) complexes, appear. The solid phase contains K3TaOF6 and K3TaO2F4. In this case the composition of cathodic products at the first peak potentials depended on the ratio of mono- and dioxofluoro complex concentrations in the melt, as well as on the current density and melt temperature.

  According to the data of amperometric titration, the mechanism of Ta2O5 interaction with FLINAK-K3TaOF6 was suggested:

  Ta2O5+4KF=K3TaO2F4+KtaO3        (1)

  K3TaOF6+KtaO3+2KF=2K3TaO2F4     (2)

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  K3TaOF6+Ta2O5+6KF=3K3TaO2F4     (3)

  By electrolysis of FLINAK-K3TaO2F4 melt cathodic deposits containing fine grains of the tetragonal tantalum and a black tantalum compound isostructural of KtaO3 were obtained.

  The Ta2O3 solubility in FLINAK melt was studied and found to occur by reaction (1). The composition of cathodic products during the FLINAK-Ta2o5 electrolysis was similar to the composition of cathodic deposits obtained from FLINAK-K3TaO2F4 melt.