Structure and Properties of the LiF-NaF-K_2NbF_7 Melts

摘要

The physico-chemical analysis of the system LiF-NaF-K_2NbF_7, based on the phase diagram, density, surface tension was performed. The system forms a simple eutectic phase diagram with the co-ordinates of the ternary eutectic point 16 mole % LiF, 30 mole % NaF, 54 mole % K_2NbF_7 and 585℃. The measurement of density in the investigated system has shown that a significant ternary interaction exists in the melts, which can be ascribed to the formation of the [NbF_8]~(3-) anions. Similarly, from the surface tension measurements in the investigated melt it follows that the formation of the complex anion [NbF_8]~(3-) is most probable in the melts of this system. The deposition of Nb from the LiF-NaF-K_2NbF_7 melts takes place via the reversible, diffusion-controlled two-step reduction mechanism [NbF_7]~(2-) + e~- => [NbF_6]~(2-) + F~-, [NbF_6]~(2-) + 4e~- = Nb(0) + 6F~-. In the presence of purposely added Na_2O the formation of the [NbOF_5]~(2-) and/or [NbO_2F_3]~(2-) oxyfluorocomplexes takes place, depending on the content of O~(2-) anions. Good experimental conditions for niobium deposition process can be expected when n_O_(Nb(V)) < 0.7. At n_O/N_(Nb(V)) > 1 inhomogeneous niobium deposits containing niobium oxide solid solution, even a non-metal deposit could be obtained. From the voltammetric and chronopotentiometric measurements in the system LiF-NaF-K_2NbF_7-Na_2O the formation Gibbs energies of liquid K_2NbF_7, K_2NbOF_6, and K_2NbO_2F_3 at the temperature of 750℃ were calculated. The values Δ_fG~0(K_2NbF_7) = (-1340+-25) kJ/mol, Δ_fG~0(K_2NbOF_5) = (-1526+-16) kJ/mol, and Δ_fG~0(K_2NbO_2F_3) = (-1100+-100) kJ/mol have been obtained. The last two values may, however, not be exactly correct since the product of the electrochemical reduction in these cases was not pure niobium, but niobium with solid solution of oxygen.