Thermodynamics of the YAl–YAl2 system

The emf method using solid-state galvanic cells based on a CaF2 single crystal as electrolyte has been used to investigate the thermodynamics of the intermetallic couple YAl–YAl2. These represent two out of the five intermetallic compounds shown in the Y–Al phase diagram. Two kinds of cells were studied:−Pt/Y,YF3/CaF2s.c./YAl,YAl2,YF3/Pt+ and −Pt/YAl,YAl2,CaAlF5,CaF2/CaF2s.c./FeAl,a+,CaAlF5;CaF2/Pt+The cell reactions expected are, respectively, 2YAl (s)=YAl2 (s)+Y (s) and YAl (s)+Al (l, s)=YAl2 (s). The standard thermodynamic quantities ΔG°, ΔH° and ΔS° have been measured as a function of temperature for both the reactions. The ΔfH°298 of YAl has also been derived by selecting from the literature the best value of ΔfH°298 for YAl2. The value to assign to the formation of YAl have been found equal to: ΔfH°298=−107±6 kJ/mol. Moreover, ΔfS°298=2±5 J/K.mol and ΔfG°T=(−100±5)+(1.8±0.5)10−2 T kJ/mol have been estimated in the temperature range from 792 to 1007 K.