High temperature thermodynamics of the yttrium–hydrogen systems

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Abstract
The binary system, yttrium–hydrogen, has been studied at 919 K at pressures up to 1 atm by a calorimetric–equilibrium method. From the calorimetric measurements we found the enthalpy of formation of YH2 at 919 K to be −52.8±0.4 kcal mole−1. The calorimetric results are in very good agreement with some of the enthalpies reported in a recent equilibrium investigation. Comparisons between the calorimetric measurements and the available equilibrium data provide new information on the partial entropy of hydrogen both in close packed hexagonal yttrium, and in the nonstoichiometric dihydride YH2−δ. For both phases we find the partial excess entropy of hydrogen near 1000 K to be about +5 cal mole−1 K−1; this value is essentially fully accounted for by the vibrational entropy contributions of hydrogen.