Phase equilibria in the molten system KF-K2NbF7-Nb2O5 were investigated up to 20mol% of Nb2O5 by thermal analysis and differential scanning calorimetry. In the binary and ternary systems oxofluoroniobate compounds are formed. They were evidenced by comparison of experimetal X-ray diffraction patterns with those in the international X-ray diffraction database (PDF-2). In the experimental compostion range the phase diagram of the molten KF-K2NbF7-Nb2O5 system consists of five crystallisation fields corresponding to KF, K3NbF8, K2NbF7, K2NbO3F and K3NbOF6.
The surface tension of the molten system LiF-NaF-K2NbF7 has been determined using the maximum bubble pressure method. The error in the surface tension measurement was estimated to be ±1%. Based on the obtained data the composition dependence of the surface tension and of the surface tension excess of the investigated system was calculated. The results are discussed in term of the anionic composition. From the surface tension excess results the negative deviation from the ideal behavior of the studied system.