Thermodynamic Properties and Phase Equilibria in Ba–Sn Alloys

Abstract

The mixing enthalpies for liquid Ba–Sn alloys were determined by isoperibolic calorimetry at 1300 K over the entire composition range. Large exothermic mixing effects ( ∆Hmin = –54.8 ±± 1.8 kJ/mole at xBa = 0.56) were found. They are indicative of strong interaction between different components and of short ordering in the liquid alloys. The thermochemical properties of the melts and barium stannides and the phase equilibria in the Ba–Sn system were used to calculate the activities of components, molar fractions of associates, and formation enthalpies and entropies for liquid BaxSn1–x alloys with the ideal associated solution (IAS) model. Two associates, BaSn and Ba2Sn, were selected for the calculation. The calculated activities of components in the Ba–Sn melts show very large negative deviations from the ideal solution, which is consistent with their thermochemical properties. The maximum concentration of each associate is approximately 0.65 for respective compositions. The calculated formation enthalpies for BamSnn intermetallics are quite high exothermic values that agree well with the published data for BaSn3 and only qualitatively for Ba2Sn. The formation enthalpy for the associate agrees with that for the respective BaSn intermetallic and is slightly less exothermic for Ba2Sn. This all indicates that the bonding energies between different atoms in the respective intermetallics and melts are close to each other and quite high. The formation entropy for the Ba2Sn associate is lower and that for BaSn is higher in magnitude. This demonstrates different degrees of ordering in BaSn and Ba2Sn associates and intermetallics. This may be due to different changes in the oscillation frequencies of atoms and other factors. The calculated liquidus curve of the Ba–Sn phase diagram agrees with data for the Sn–Sr system.