Thermodynamics of Hydrogen Solution and Hydride Formation in Pd−Mn Alloys. 1. Disordered Alloys and a Correlation Effect

The thermodynamics of H₂ solution and hydride formation/decomposition have been determined by reaction calorimetry (303 K) for disordered face centered cubic (fcc) Pd−Mn alloys. This alloy system belongs to the expanded lattice category which predicts that and ΔH_plat for H₂ absorption should be more exothermic than those for Pd; the experimental results are that the former is more exothermic, at least at the higher Mn contents, but the latter is not. There is a regular decrease in the H capacity (at p_H2 = 0.2 MPa) with atom fraction Mn. A linear dependence of log p_H2 upon H content is found in the single hydride phase for all of these alloys suggesting that ΔH_H and ΔS_H are also linear functions of r in this region. This is confirmed using the Pd_0.875Mn_0.125 alloy which has no two-phase region (303 K). It is shown for the Pd_0.875Mn_0.125 alloy and for Pd that the changes of partial enthalpies and entropies with H content are correlated so as to minimize changes of μ_H.