A thermodynamic study of the hydrogenation of the pseudo-binary Mg6Pd0.5Ni0.5 intermetallic compound

Hydrogenation thermodynamic properties of a new pseudo-binary Mg6Pd0.5Ni0.5 intermetallic compound (IMC) are presented. This IMC disproportionates reversibly on hydrogenation into MgH2, Mg5Pd2 and Mg2NiH4 compounds. Pressure–Composition–Temperature measurements in the temperature range 590–650 K have been acquired. The enthalpy and entropy of the hydrogenation reaction are respectively, −63 ± 3 kJ/molH2 and −114 ± 4 J/K molH2. They agree well with those calculated from the enthalpy and entropy of formation of the several phases involved in the disproportionation reaction. In particular, the Miedema model has been used to calculate the standard enthalpy of formation of the several Mg–Pd IMC's, ranging from −21.8 to −61.5 kJ/mol atoms for Mg6Pd and MgPd, respectively. As for the standard entropy changes, an inverse relationship between entropy and density was used to calculate the formation entropy of the Mg–Pd IMC's.