Hydrogen solution in tetrahedral or octahedral interstitial sites in zirconium-cobalt hydrogen storage alloy: A first-principles study

Abstract

The site occupation of H atoms in ZrCo alloys was investigated using the first principle study based on pseudopotential plane-wave method. Crystal structural models of ZrCo and its hydrides were established, and the electronic structures and bonding characteristics of ZrCo and its hydrides were evaluated to obtain their total energy, site occupation energy, band structure, the density of states (DOS), and Mulliken population distribution. In the ZrCo crystals, the inter-tetrahedron space is larger than the inter-octahedron space, the bond strength of Co–H is higher than that of Zr–H in unit cells of metallic hydrides, and the bond strength of Co–H(T) is higher than that of Co–H(O). Therefore, H atoms tend to occupy the inter-tetrahedron space in ZrCo hydrides owing to the excellent structural stability of these hydrides. Further studies of ZrCo hydrides electronic structures revealed that s orbital electrons in H atoms and d orbital electrons in Co atoms in ZrCo hydrides are in anti-bonding states, the bandwidth of the Fermi energy (Ef) and the pseudo-gap energy is reduced, resulting in degraded stability of ZrCo hydrides, thus facilitating hydrogen release by the hydrides.