First-principles calculations of LaNi_4Al-H solid solution and hydrides

摘要

First-principles calculations on the crystal and electronic structures of LaNi_4AlHY (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 LaNi4Al 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 LaNi4Al. 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.