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)

Abstract

Isothermal hydrogenation performances of intermetallic Mg<SUB>2-y</SUB>Pr<SUB>y</SUB>Ni<SUB>4</SUB> alloys with <i>y</i> = 0.6, 0.8 and 1.0 reported by Terashita<i>et al</i>.were analyzed on the basis of statistical thermodynamics under a simplifying<i>a priori </i>assumption of constant nearest neighbourH-H interaction<i>E</i>(H-H) in a given phase at arbitrary <i>T</i> aiming at characterizing basic aspects of state of H atoms in the interstitial sites in H-storage alloy. To fulfill this <i>a priori</i> 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 Mg<SUB>2-y</SUB>Pr<SUB>y</SUB>Ni<SUB>4 </SUB>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 Mg<SUB>2-y</SUB>Pr<SUB>y</SUB>Ni<SUB>4</SUB>. On the other hand, hypo-stoichiometric M<SUB>4</SUB>H<SUB>3</SUB> type hydride of Mg<SUB>2-y</SUB>Pr<SUB>y</SUB>Ni<SUB>4</SUB> 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 <i>E</i>(H-H) = 0 for any Mg<SUB>2-y</SUB>Pr<SUB>y</SUB>Ni<SUB>4</SUB>examined. 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 <i>p</i>(H<SUB>2</SUB>) plateau during H-charging.