High temperature thermodynamics of palladium–hydrogen. II. Temperature dependence of partial molar properties of dilute solutions of hydrogen in the range 500–700 K

The thermodynamic properties of hydrogen in dilute solutions in palladium have been studied in the temperature range 500–700 K both by the equilibrium method and by calorimetry. In this range the partial enthalpies and excess entropies of hydrogen increase very significantly with temperature. This suggests the existence of a ’’soft’’ transition for the dissolved hydrogen atoms. The experimental values of the partial excess entropies of hydrogen in palladium may be explained by assuming occupancy by hydrogen of both tetrahedral and octahedral interstitial sites at room temperature or by the modification of the vibrational spectrum of palladium. The further increase of the excess entropy above room temperature may be due to the gradual accumulation of a communal entropy of R between room temperature and 700 K.