THERMODYNAMICAL PROPERTIES AND PHASE EQUILIBRIUMS IN ALLOYS OF THE Bi–La SYSTEM

The method of isoperibolic calorimetry was used for the first time to determine the partial and integral enthalpies of mixing of melts of the Bi–La system at a temperature of 1220±2 K in a wide range of compositions. It was established that the minimum value of the enthalpy of mixing of melts of this system −118±14 kJ/mol falls on the composition with xBi = 0.4; ∆H̅ = −401±73; ∆H̅ = −212±49 kJ/mol. It was established that almost all known, agree both with ours and with each other, despite the fact that they were not determined by the direct EMF method. To confirm the reliability of the obtained data and search for general patterns of thermodynamic characteristics of alloy formation of the Bi–La system, it was considered as a member of the Bi–Ln(Ln-lanthanide) system series. For this purpose, the enthalpies of formation and Tm intermetallics LnBi, as well as the minimum values of the enthalpy of mixing of melts (ΔHmin) of melts, relative differences of molar radii and electronegativities of components of the Bi–Ln systems and their dependences on the ordinal number of the lanthanide are constructed. It is shown that all dependences, except for the differences of electronegativities of components, are symbiotic with each other. This indicates that the thermodynamic properties of all phases of the Bi–Ln systems are determined by the size factor. Іt was found that the Tm of the YbBi compound is lower than that of compounds of metals located near Yb. Since the Tm of compounds is related to the energy of interatomic interaction, it can be assumed that the enthalpy of formation of YbBi monobismuthide is less exothermic than that determined in one work (-91 kJ/mol). T m of the EuBi compound should also be lower than that of compounds of systems located near Eu and equal to 1200 K. But this and other parameters predicted by us must be clarified by methods of physicochemical analysis. It is shown that the predicted values of the minimum enthalpies of mixing of unstudied melts of the Bi–Ln systems for light REMs are close to the enthalpies of formation of their monobismuthides, and for heavy REMs they are less exothermic than their ∆fH, as was established for the Bi–Yb system.