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  • Author: Vladimír Daněk x
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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.

Formation of oxyfluoroniobium compounds in the binary systems KF-Nb2O5 and K2NbF7-Nb2O5 and in the ternary system KF-K2NbF7-Nb2O5 has been expected. Therefore the phase equilibrium and surface tension of the above systems have been determined. The three systems have only been investigated up to 20 mol% Nb2O5 because of its limited solubility. The obtained results have confirmed the formation of oxyfluoroniobium compounds.


The density of the melts in the system LiF–NaF–K2NbF7 has been measured using an Archimedean method. Based on the measured density values, the molar volumes, partial molar volumes and excess molar volumes of the melts were calculated. 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 different complex anions.


The density and surface tension of melts of the systems CaO-FeO-Fe2O3 MgO at the temperature 1623 K, CaO-FeO-Fe2O3-ZnO at 1573 K, and CaO-Fe2O3-Cu2O at 1573 K were determined using the maximum bubble pressure method. The molar volume, the excess molar volume, and the excess surface tension were calculated on the basis of the obtained data. From these properties information on the interactions of components and possible chemical reactions between them was obtained. Due to the absence of silica and the low concentration of other network-forming oxides, only isolated FeO45− tetrahedra and the CaO·FeO ionic pairs are formed in these basic melts, the donor of the oxygen atoms being either CaO, FeO, or both CaO+FeO oxides. Even the observed ternary interactions may be attributed to the formation of the anions FeO45− only.