Potassium fluorotantalate in solid, dissolved and molten conditions

Calorimetric study of melts in the system KF - K2TaF7

“…Complexes with higher molar ratio of tantalum to fluorine with respect to TaF 8 3-and TaF 7 2-may also be created in the melt. In accord with some spectroscopic investigations [1,25] the formation of TaF 6 -depending on the amount of the KF in the system and the temperature may be expected.…”

“…The shifting of the strong interaction to the K 2 TaF 7 richer side of the system shows that except association processes resulting in formation of the TaF 8 3-anions in this system also other processes take place. In accord with some spectroscopic studies [1,25] formation of TaF 6 -anions may be assumed.…”

“…The melting temperatures of K 3 TaF 8 published by different authors varied from 1049 to 1053 K. Some discrepancies exist in the interpretation of the diagram in the K 2 TaF 7 rich side and in the interpretation of melting of K 2 TaF 7 itself, due to complicated character of reactions which accompany this process. According to Agulyansky and Bessonova [5] and Agulyansky [1,6] partial decomposition of the K 2 TaF 7 occurs at 1003 K due to the incongruent melting, yielding molten KTaF 6 and solid K 3 TaF 8 . The rest of the K 2 TaF 7 melts, probably by some different mechanism, in a separate step at 1019 K. Finally, K 3 TaF 8, , produced by the incongruent decomposition at 1003 K, melts at 1051 K. Recently a new interpretation has been published, based on DSC and high temperature neutron diffraction [7].…”

“…While structure and properties of crystalline complex fluorotantalates are in many cases known, (recently comprehensive review of fluorometalates including fluorotantalates has been published [10]), the understanding of the ionic structure of melts is still in development. According to Agulyansky [1,6] three conceptual steps can be discerned in definition of the ionic structure of fluoride melts containing tantalum. The first attempt, based on analyses of melting diagrams and available estimated thermodynamic data assumes that in molten systems containing alkali-metal fluorides, tantalum forms complex ions in which the coordination number of tantalum increases up to 8.…”

“…The investigation showed the tendency of tantalum to reduce its coordination number when moving from crystalline to molten state. Accordingly, the properties of melts depend on the equilibrium between two types of complex ions, TaF 7 2-and TaF 6 -. Frequently, the ionic compositions of melts are estimated by analysis of melting diagrams and physicalchemical properties like density, surface tension, viscosity and electrical conductivity.…”