The effect of Pd on hydride formation in Zr(Pd_xM_1−x)₂ intermetallics where M is a 3d element

We report how the substitution of 3d metal atoms M by Pd in Zr(Pd_xM_1−x)₂ pseudobinary compounds affects the stability of the corresponding hydrides. For the lighter elements in the M=3d metal row, V and Cr, the Pd substitution creates less stable hydrides while it yields almost no change in the stability of the hydrides for Mn, of the middle of the 3d row. We further demonstrate that the two factors determining the stability of the hydrides of these compounds are the heat of formation of the compound, ΔH_alloy, and its crystal volume, V. Using a combination of these two parameters we predict that the substitution of the heavier 3d metals Fe and Co by Pd stabilizes the Zr(Pd_xM_1−x)₂ hydrides, i.e. they would require lower hydrogenation pressures than those necessary for ZrFe₂ and ZrCo₂ hydride formations. These findings support other authors’ theoretical prediction for ZrPdFe hydride formation. The empirically fitted constants that link ΔH_alloy and V are related to the energies needed to expand or compress the crystal lattice and thus have basic physical meaning. We further evaluate the critical temperatures, T_c, for Zr(Pd_xMn_1−x)₂-H₂ systems from the experimental pressure hysteresis in the absorption-desorption loops. This enables us to show that the electronic contribution to the H-H interaction in the ZrM₂ hydride is unexpectedly neutral for M=Mn, it is positive, i.e. repulsive, for the lighter 3d element M=Cr, and surprisingly negative, i.e. attractive, for the binary PdH_0.6 hydride.