Calorimetric study of melts in the system KF-K2TaF7

Enthalpy increment measurements on melts in the system KF-K2TaF7 were carried out by drop calorimetry at temperatures between 298 K and 1063, 1103 and 1143 K for selected compositions. Heat capacities of the melted mixtures and enthalpies of mixing have been determined. Careful calorimetric experiments showed small but distinct non-ideality of the melt. The molar heat capacity of melt exhibits small positive divergence from additivity. The molar enthalpy of mixing shows negative deviation from ideality which decreases with increasing temperature. The thermal effect at mixing was assigned predominantly to association reactions producing more complex fluorotantalate anions. The formation of complex anions with lower coordination number of Ta may not be excluded.