Vaporization thermodynamics of Pd-rich intermediate phases in the Pd-Yb system

The vaporization thermodynamics of several intermediate phases in the Pd-Yb system was investigated by means of vaporization experiments performed under Knudsen conditions (KEML, Knudsen Effusion Mass Loss). The following thermal decomposition processes were studied in the overall temperature range 819-1240 K and their enthalpy changes determined: 4 PdYb(s) = Pd4Yb3(s) + Yb(g); 5/3 Pd4Yb3(s) = 4/3 Pd5Yb3(s) + Yb(g); 21/13 Pd5Yb3(s) = 5/13 Pd21Yb10(s) + Yb(g); 1/3 Pd21Yb10(s) = 21/9 Pd3Yb(s) + Yb(g). Additional measurements were performed by KEMS (Knudsen Effusion Mass Spectrometry) on a Pd-rich two-phase sample, which allowed to detect both Yb(g) and Pd(g) in the vapor phase and to determine the atomization enthalpy of the Pd3Yb phase (Pd-rich composition boundary, Pd3.08Yb0.92): Pd3.08Yb0.92(s) = 0.92 Yb(g) + 3.08 Pd(g). The enthalpy of formation of this compound was thereafter determined as −68 ± 2 kJ/mol at. and, by combining this value with the decomposition enthalpies derived by KEML, the enthalpies of formation of the studied Pd-Yb intermediate phases were evaluated (kJ/mol at.): −75 ± 4 (Pd21Yb10), −75 ± 3 (Pd5Yb3), −73 ± 3 (Pd4Yb3), and −66 ± 3 (PdYb). A modified version of the Pd-Yb phase diagram is also reported, re-drawn on the basis of literature data and of new experimental information recently become available.