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The thermodynamic characterization of an important intermetallic compound-H 2 system, ZrNi-H 2 , has been analyzed employing an experimental approach of a calorimeter coupled to a high precision volumetric device. The studies have been carried out over a 350 to 550 K temperature range where special attention was paid to the validity of the heat measurements in the low pressure domain. Strict control of hydride phase growth has been ensured in order to eliminate problems associated with hysteresis and with the presence of the poor heat conductivity of the powder sample. This work establishes that meaningful heat measurements become possible above 463 K and also that the system is no longer sensitive to the activation and the annealing treatment above 426 K. The measured enthalpies for the ␤ → ␥ transformation were determined as Ϫ34.3 Ϯ 0.5 kJ/mol H and allow the conclusion to be drawn that there is no significant temperature dependence of the enthalpy of transformation over a range of ⌬T ϭ 180 ЊC. Finally, using the present results together with previous investigations it is clearly demonstrated that the phase diagram of the ZrNi-H system must be re-examined.

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