Structural and thermodynamic aspects of niobium deposition in the system LiF-KF-K2NbF7

The complex physico-chemical analysis of the system LiF-KF-K2NbF7, based on the phase diagram, density, surface tension, and viscosity measurements, w as performed. The results proved the presence of the congruently melting co mpound K3NbF8 even in the ternary melts. The calculated co-ordinates of the two ternary eutectic points are: e(1): 22.3 mole % LiF, 9.4 mole % KF; 68. 3 mole % K2NbF7; t(e)= 649 degreesC; e(2): 45.5 mole % LiF, 51.7 mole % KF; 2.8 mole % K2NbF7; t(e)= 486 degreesC. The probable inaccuracy in the calc ulated ternary phase diagram is 4.7 degreesC. The degree of thermal dissociation of the additive compound K3NbF8 alpha (0 ) = 0.55 at 1100 K, calculated from the density measurement agrees well wit h the value determined from the analysis of the phase diagram (alpha (o) = 0.44) as well as from the viscosity measurement (alpha (o) = 0.45 at 1100 K ) and refers to the pronounced thermal dissociation of this compound at mel ting. The formation of K3NbF8 was confirmed by all of the followed physico- chemical parameters and it is the general feature of this system. The deposition of Nb from the LiF-KF-K2NbF7 melts takes place via the rever sible, diffusion-controlled two-step reduction mechanism [NbF7](2-) + e(-) = [NbFs(6)](2-) + F- [NbF6](2-) + 4e(-) = Nb(0) + 6F(-) In the presence of oxygen in the melt the formation of the [NbOF5](2-) or/a nd [NbO2F](-) oxyfluorocomplexes takes place, depending on the concentratio n of O2- anions. Good experimental conditions for niobium deposition proces s can be expected when n(O)/n(Nb(nu)) < 0.7. At n(O)/n(Nb(<nu>)) > 1 inhomo geneous niobium deposits containing niobium oxide solid solution, even a no n-metal deposit could be obtained. The diffusion coefficients of electroact ive species were calculated from the results of electrochemical measurement s. (C) 2000 Elsevier Science B.V. All rights reserved.