Hydrogen in Laves phaseZrX2(X=V,Cr, Mn, Fe, Co, Ni) compounds: Binding energies and electronic and magnetic structure

Abstract

The effects of hydrogen on the ground-state electronic and magnetic structures of ${\mathrm{AB}}_{2}$ (A=Zr; $B=\mathrm{V},$ Cr, Mn, Fe, Co, Ni) Laves phase compounds were investigated by first-principles local-density-functional calculations. We calculated the relative stability of atomic hydrogen at various interstitial tetrahedral sites formed by two A atoms and two B atoms $(2A2B$ site), one A atom and three B atoms $(1A3B$ site), and four B atoms $(4B$ site). We find that (i) for ${\mathrm{ZrV}}_{2}$ and ${\mathrm{ZrCr}}_{2},$ hydrogen prefers the site with the largest interstitial hole size (i.e., $2A2B$ site); (ii) for ${\mathrm{ZrMn}}_{2},$ ${\mathrm{ZrFe}}_{2},$ and ${\mathrm{ZrCo}}_{2},$ the hydrogen binding energies at the $1A3B$ site become comparable to or even lower than those at the $2A2B$ site once the antibonding states of B atoms become progressively occupied. In H-free ${\mathrm{ZrFe}}_{2},$ we found a large magnetic moment $(0.5{\ensuremath{\mu}}_{\mathrm{B}})$ at the Zr site, which is coupled antiparallel to the moment at the Fe site $(1.9{\ensuremath{\mu}}_{\mathrm{B}}).$ The hydrogen absorption does not have a strong effect in suppressing the magnetic moments of atoms closest to the absorbed hydrogen but increases the magnetic moments of atoms not having hydrogen as the nearest neighbor. This increase in the moment is partly due to the H-induced lattice expansion.