Thermodynamics of the YAl-YAl2 system

The emf method using solid-slate galvanic cells based on a CaF2 single crys tal as electrolyte has been used to investigate the thermodynamics of the i ntermetallic couple YAl-YAl2, These represent two out of the five intermeta llic compounds shown in the Y-Al phase diagram. Two kinds of cells were stu died: (-)Pt/Y, YF3/CaF(2)s.c./YAl, YAl2, YF3/Pt(+) and (-)Pt/YAl, YAl2, CaAlF5, C aF2/CaF(2)s.c./Fe(Al, a(+)), CaAlF5; CaF2/Pt(+) The cell reactions expected are, respectively, 2YAl (s) = YAl2 (s) + Y (s) and YAl (s) + Al (1, s) = YAl2 (s). The standard thermodynamic quantities D elta G degrees, Delta H degrees and Delta S degrees have been measured as a function of temperature for both the reactions. The Delta(f)H degrees(298) of YAl has also been derived by selecting from the literature the best val ue of Delta(f)H degrees(298) for YAl2. The value to assign to the formation of YAl have been found equal to: Delta(f)H degrees(298) = -107 +/- 6 k/J.m ol. Moreover, Delta(f)S degrees(298) = 2 +/- 5 kJ/mol and Delta(f)G degrees (T) (-100 +/- 5) + (1.8 +/- 0.5)10(-2) T kJ/mol have been estimated in the temperature range from 792 to 1007 K. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.