Determination of dissociation degrees of K3NbF8 and K3TaF8 by thermodynamic analysis of subsystems of the KF-K2NbF7 and KF-K2TaF7 systems

A special form of the LeChatelier-Shreder equation describing the equilibrium between the crystalline phase and the melt in system A-AB in which the substance AB partially dissociates upon melting was applied to systems KF-K3NbF8, K2NbF7-K3NbF8 and to KF-K3TaF8, K2TaF7-K3TaF8 subsystems of the binary systems KF-K2NbF7 and KF-K2TaF7 in which the additive compounds K3NbF8 and K3TaF8 are formed. Using the phase diagram of the system KF-K2NbF7 determined by McCawley and Barclay (1971) and the values of the fusion enthalpy of K3NbF8 taken from literature, the intervals of the dissociation degree values of K3NbF8 for both branches of the liquidus curve of K3NbF8 were calculated. The calculated values of the dissociation degree depend on the coordinates of the liquidus curve of K3NbF8 of the pertinent phase diagram, on its used branch and section, and on the value of the fusion enthalpy of K3NbF8. For the measured fusion enthalpy of K3NbF8 (57 kJ mol−1), a common interval of the dissociation degree values of K3NbF8 for both branches of the liquidus curve of K3NbF8 is 0.71–0.72. Similarly, intervals of the dissociation degree values of K3TaF8 for both branches of the liquidus curve of K3TaF8 were calculated using the phase diagram of the system KF-K2TaF7 determined by Boča et al. (2007) and the measured fusion enthalpy of K3TaF8 ((52 ± 2) kJ mol−1). The error of the determination of the fusion enthalpy of K3TaF8, the common interval of the dissociation degree values of K3TaF8 for both branches of the liquidus curve of K3TaF8 is 0.68–0.69.