THERMODYNAMIC PROPERTIES OF MELTS OF THE Eu-Ge AND Al-Eu-Ge SYSTEMS

Abstract

The thermochemical properties of the melts of the Eu–Ge and Al–Eu–Ge systems at 1200–1400 K were determined for the first time by the method of calorimetry. It was established that the minimum value of the enthalpy of mixing of the melts of the Eu–Ge system is -49.1±4.4 for an alloy with xGe = 0.45, ΔH̅∞Eu = -145.7 ± 22.3; ΔH̅∞Ge = −166.8 ± 19.8 kJ/mole at T = 1400 K. The curve of enthalpy of mixing of the studied melts is almost symmetrical, which correlates with the behavior of alloys of this and similar systems in the solid state. The obtained data make it possible to consider the entire spectrum of Ge–Ln systems (lanthanides) and to explain what determines the thermodynamic properties of melts of the Eu–Ge system, in particular, and Ge–Ln, in general. Using the defined thermochemical properties of melts and the known phase diagram of the Eu–Ge system, the Gibbs energy, enthalpy, and entropy of formation of melts, associates in melts, and compounds of the Eu–Ge system were optimized and calculated according to the ideal associated solution (IAS) model. The calculated activities of the components in the melts of this system show large negative deviations from ideal solutions. Using the obtained thermochemical data, the temperatureconcentration dependences of Gibbs energies, enthalpies and entropies of the formation of melts and intermetallics, and the coordinates of the liquidus curve of the state diagram of the Eu–Ge system were also calculated according to the IAS model. The calculated liquidus curve and the one known from experimental data do not fully agree with each other. Comparison of ΔHmin of melts with enthalpies of formation, temperatures of melting Ln5Ge3 compounds, and differences in molar volumes and electronegativities of components of Ge–Ln systems depending on the serial number of Ln showed that they are correlated with each other, and ΔHmin of melts and Ln5Ge3 compounds differ little from each other. In addition, all dependences, with the exception of Δχ, are symbiotic and monotonic, with the exception of compounds and melts of binary Ge–Eu(Yb) systems. It was established that the integral and partial enthalpies of mixing of melts of radial sections with xEu/xGe = 0.85/0.15 and 0.3/0.7 of the Al–Eu–Ge system at 1400 K are exothermic. When aluminum is added to GexEu1-x melts, the thermal effect of its dissolution in the first section initially decreases, and then it increases, and in the second – it increases. This is due to the breaking of strong Eu–Ge bonds and the formation of lower energy bonds between Al and Eu. The thermodynamic properties of melts of the Al–Eu–Ge system were calculated using the “geometric” models and the Redlich-Kister-Mujianu model based on similar data for limiting binary systems. It is shown that those calculated according to the Redlich-Kister-Mujianu model with the ternary contribution L = -220 kJ/mole are in the best agreement with the experimental data. Therefore, the same model was used to calculate other thermodynamic properties of melts of the Al–Eu–Ge system.