Statistical Thermodynamic Analysis for Isothermal Hydrogenation Performances of Mg<SUB>2-</SUB><SUB>y</SUB>Pr<SUB>y</SUB>Ni<SUB>4</SUB>Intermetallics (<i>y</i> = 0.6, 0.8, 1.0)

Isothermal hydrogenation performances of intermetallic Mg2-yPryNi4 alloys with y = 0.6, 0.8 and 1.0 reported by Terashita et al.were analyzed on the basis of statistical thermodynamics under a simplifyinga priori assumption of constant nearest neighbourH-H interactionE(H-H) in a given phase at arbitrary T aiming at characterizing basic aspects of state of H atoms in the interstitial sites in H-storage alloy. To fulfill this a priori assumption, number θ of available interstitial sites per metal atom was chosen by preliminary search attempt at the onset of the statistical thermodynamic analysis. Primary H solution in Mg2-yPryNi4 was analyzed by the model with θ = 0.15. The chosen value 0.15 for the model analysis was close to be 1/6 (≈ 0.167) which was half of 1/3 (=[Mg + Pr]/[Mg + Pr + Ni])implying that about half of the (Mg + Pr)-related interstitial sites were provided as the available sites for occupation by H atoms in the primary H solution of Mg2-yPryNi4. On the other hand, hypo-stoichiometric M4H3 type hydride of Mg2-yPryNi4 was analyzed by the model with θ = 0.75 and θ' = 0.333 where ' refers to the lower limiting composition of the phase. This model yielded situation with E(H-H) = 0 for any Mg2-yPryNi4examined. Chosen value of θ' = 0.333 appeared to imply that the filling of Ni-related interstitial sites by H atoms started after preferential full occupation of the (Mg + Pr)-related interstitial sites by H atoms in the two-phase equilibrium range at invariable p(H2) plateau during H-charging