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

Abstract

Abstract The complex physico-chemical analysis of the system LiF-KF-K2NbF7, based on the phase diagram, density, surface tension, and viscosity measurements, was performed. The results proved the presence of the congruently melting compound K3NbF8 even in the ternary melts. The calculated co-ordinates of the two ternary eutectic points are: e1: 22.3 mole % LiF, 9.4 mole % KF; 68.3 mole % K2NbF7; te = 649 °C; e2: 45.5 mole % LiF, 51.7 mole % KF; 2.8 mole % K2NbF7; te = 486 °C. The probable inaccuracy in the calculated ternary phase diagram is 4.7 °C. The degree of thermal dissociation of the additive compound K3NbF8 α0 = 0.55 at 1100 K, calculated from the density measurement agrees well with the value determined from the analysis of the phase diagram (α0 = 0.44) as well as from the viscosity measurement (α0 = 0.45 at 1100 K) and refers to the pronounced thermal dissociation of this compound at melting. 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 reversible, diffusion-controlled two-step reduction mechanism [ NbF 7 ] 2− + e − = [ NbF 6 ] 2− + F − [ NbF 6 ] 2− + 4e − = Nb (0) + 6 F − In the presence of oxygen in the melt the formation of the [NbOF5]2− or/and [NbO2F]− oxyfluorocomplexes takes place, depending on the concentration of O2− anions. Good experimental conditions for niobium deposition process can be expected when nO/nNb(V) 1 inhomogeneous niobium deposits containing niobium oxide solid solution, even a non-metal deposit could be obtained. The diffusion coefficients of electroactive species were calculated from the results of electrochemical measurements.