Thermodynamics of solutions of hydrogen in palladium–manganese alloys. I. Pd3Mn from 555 to 909 K

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Abstract
The thermodynamic properties of dilute solutions of hydrogen and deuterium in Pd3Mn have been determined by a calorimetric-equilibrium method between 555 and 909 K. This alloy undergoes an order–disorder transition at 750–800 K, i.e., in the temperature range of our measurements. In this temperature region the thermodynamic properties of hydrogen change very significantly. The partial entropies of hydrogen in the alloy are quite low compared to the values in pure palladium; this points toward a highly nonrandom distribution of the hydrogen atoms among the available interstitial sites. From the isotope effect the vibrational frequencies of hydrogen are calculated to be 600 cm−1 at 555 K, 650 cm−1 at 700 K, 770 cm−1 at 777 K, and 860 cm−1 at 807 K, i.e, increasing significantly with increasing temperature. Note in particular the increase in frequency in the transition region. This change correlates with a reduction in the partial entropy of hydrogen of about 1.8 cal K−1 mol−1 from the ordered to the disordered alloy.