First-principles calculations of LaNi 4Al–H solid solution and hydrides

Abstract

First-principles calculations on the crystal and electronic structures of LaNi 4AlH x ( x = 1, 2, 3, 4, 4.5, 5 and 7) solid solution and hydrides have been performed by employing the plane-wave pseudo-potential method. Their equilibrium structures have been determined based on the calculated relative stabilities of hydrogen occupation sites in LaNi 4Al and the rule of H–H separation to be no less than 0.21 nm, as well as the full geometry optimization. The covalency and ionicity analysis shows that the interaction between H and Ni is covalent and ionic; between H and La mainly ionic; between H and Al for x = 1, 2, 3 and 4 mainly ionic and for x = 4.5, 5 and 7 both covalent and ionic. The change of BO H - Ni s ( or Ionicity H – La s ) with x can be a phenomenological indication of the experimental phase transformation for hydrogen solubility in LaNi 4Al. The comparison of the calculated stability of hydrogen at interstices in LaNi 4Al and LaNi 5 shows that the possible hydrogen occupation sites in LaNi 4Al are fewer than those in LaNi 5. This contributes to the decrease of hydrogen storage capacity for LaNi 4Al, which is illustrated by comparing the electron density distribution of LaNi 4AlH 7 with LaNi 5H 7. Except for H–H repulsions, the change of absorption energy with hydrogen content, arising from the fact that an H atom tends to keep away from an Al atom, elucidates the limited maximum hydrogen content in LaNi 4Al.