Thermodynamics of solutions of hydrogen in palladium–manganese alloys. II. Pd to PdMn

Abstract

The thermodynamic properties of dilute solutions of hydrogen in alloys of palladium with up to 50 at. % manganese have been studied by a combined calorimetric-equilibrium method between 555 and 909 K. The results are compared with corresponding data for hydrogen in pure palladium and in the alloy Pd3Mn; the latter undergoes an order–disorder transformation at 750–800 K. In some cases data also were obtained for deuterium; a comparison between the properties of the two hydrogen isotopes provides approximate information on the vibrational frequency of hydrogen in the alloy. For the disordered solid solution of manganese in palladium we find little change in the vibrational frequency with alloy composition. Particular interest is attached to the ’’decomposition’’ of the excess Gibbs energy into its enthalpy and entropy ’’parts’’. Our results show that for alloys up to 25 at. % manganese increasing manganese content. The dependence of the enthalpy on temperature also becomes increasingly sharp and there is a corresponding large increase in the excess entropy of hydrogen. These changes are attributed to increased occupation by the hydrogen atoms at higher temperatures of energetically less favorable sites.