Thermodynamics of Hydrogen Solution and Hydride Formation in Pd−Mn Alloys. 2. Ordered Alloys

There are marked differences in H₂ solubilities between ordered and disordered Pd−Mn alloys with the largest difference found between the L1₂ and the disordered form of the Pd₃Mn alloy. The thermodynamics of H₂ solution have been determined for the L1₂ form, the long-period superstructure (lps), and the disordered forms of the Pd_0.80Mn_0.20 and Pd_0.75Mn_0.25(Pd₃Mn) alloys. Relative partial molar enthalpies and entropies were determined mainly by reaction calorimetry over the range of H contents accessible from p_H2 ≈ 10 Pa to ≈ 0.3 MPa (303 K). The enthalpies for absorption of H₂ are more exothermic over most of the range of H contents for the L1₂ forms of the Pd₃Mn and Pd_0.80Mn_0.20 alloys than for their other forms. The reaction enthalpies are constant across a relatively wide range of H contents for the L1₂ form of the Pd_0.80Mn_0.20 and Pd₃Mn alloys indicating that there are two-phase coexistence regions (303 K). The H−H attractive interaction, which leads to hydride formation, is much greater for the L1₂ than for the other forms of the Pd₃Mn alloy and for Pd itself. It has been found that the H−H interaction always decreases in magnitude and, accompanying this, the THS (terminal hydrogen solubility) always increases by alloying Pd.¹ The L1₂ ordered Pd₃Mn alloy is an exception to this, and therefore, the generalization about THS must be restricted to disordered face centered cubic (fcc) Pd alloys.