Calorimetric enthalpies of formation and decomposition of hydrides of ZrMn2, ZrCr2, and related systems

Integral enthalpies of absorption and desorption of hydrogen by hyperstoichiometric ZrMn2T0.8 (T = Mn, Fe, Co, Ni, and Cu) and stoichiometric ZrMn2- and ZrCr2-based alloys have been determined. The measured enthalpies range from ∼24 to ∼41 kJ/mole H2. The ΔH values for hydrides formed by the series of metallic hosts ZrMn2T0.8 are smaller than that for ZrMn2, accounting for the enhanced dissociation pressures of the ZrMn2T0.8 hydrides. In the series of ZrMn2T0.8 hydrides there is a pronounced minimum for hydride of ZrMn2Co0.8, accounting for the extraordinarily high decomposition pressure of this system. Site occupancies, provided by published neutron diffraction studies, were used to calculate configurational entropies of ZrCr2 hydrides and related systems. Results obtained were in fair agreement with experiment.