The role of the alkaline cations in the density and volume properties of the melts MF-K2NbF7 (MF = LiF-NaF, LiF-KF and NaF−KF)

The density of the ternary system NaF-KF-K2NbF7 was measured by the Archimedean method. The molar volume, excess molar volume and partial molar volumes have been calculated at different temperatures. Mathematical descriptions of the concentration dependence of the density and molar volume have been generated. Obtained results have been compared with the respective quantities of the systems LiF-KF-K2NbF7 and LiF-NaF-K2NbF7. Two opposite effects influencing the volume properties have been identified. On one side an association reaction of F− and [NbF7]2− anions forming [NbF8]3− was considered resulting to the volume contraction. On other side the volume expansion was identified caused by the present alkaline cation. Overall result of these two effects is volume contraction.