Changes in electronic, magnetic and bonding properties from Zr2FeH5 to Zr3FeH7 addressed from ab initio

• Zr2FeH5 is found more cohesive and harder than Zr3FeH7.
• Ternary hydrides are harder than pristine intermetallics due to metal–H bond.
• Zr3Fe and Zr2Fe hydrogenation weakens inter-metal and favors metal–hydrogen bonding.
• Hydrogen charges range from covalent H−0.3 in Zr–Fe and less covalent H−0.5 in Zr environment.
• Onset of Fe magnetization only in Zr3FeH7 due to magnetovolume effects.

Potential hydrogen storage ternaries Zr3FeH7 and Zr2FeH5, are studied from ab initio with the purpose of identifying changes in electronic structures and bonding properties. Cohesive energy trends: Ecoh. (ZrH2) > Ecoh. (Zr2FeH5) > Ecoh. (Zr3FeH7) > Ecoh. (hypothetic-FeH) indicate a progressive destabilization of the binary hydride ZrH2 through adjoined Fe so that Zr3FeH7 is found less cohesive than Zr2FeH5. From the energy volume equations of states EOS the volume increase upon hydriding the intermetallics leads to higher bulk moduli B0 explained by the Zr/Fe–H bonding. Fe–H bond in Zr2FeH5 leads to annihilate magnetic polarization on Fe whereas Fe magnetic moment develops in Zr3FeH7 identified as ferromagnetic in the ground state. These differences in magnetic behaviors are due to the weakly ferromagnetic Fe largely affected by lattice environment, as opposed to strongly ferromagnetic Co. Hydrogenation of the binary intermetallics weakens the inter-metal bonding and favors the metal–hydrogen bonds leading to more cohesive hydrides as with respect to the pristine binaries. Charge analyses point to covalent like Fe versus ionic Zr and hydrogen charges ranging from covalent H−0.27 to more ionic H−0.5.

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