Thermodynamics of hydrogen solution and hydride formation in Pd-Mn alloys. 1. Disordered alloys and a correlation effect

The thermodynamics of H-2 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 Delta H-H(degrees) and AH(plat) for H-2 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 PH2 = 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 AHH and ASH are also linear functions of r in this region. This is confirmed using the Pd0.875Mn0.125 alloy which has no two-phase region (303 K). It is shown for the Pd0.875Mn0.125 alloy and for Pd that the changes of partial enthalpies and entropies with H content are correlated so as to minimize changes of mu(H).