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

Abstract

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 819–1240K 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±2kJ/molat. 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/molat.): −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.