Study on the interaction between hydrogen and metals of LaNi5 type hydrogen absorbing alloys by means of DV-Xα method

The electronic structures of LaNi5, LaNi4.5Co0.5, La0.6Ce0.4Ni5, LaNi5H0.2, LaNi4.5Co0.5H0.2 and La0.6Ce0.4Ni5H0.2 were studied by means of the discrete variational Xα (DV-Xα) method. The interaction between hydrogen and the hydrogen absorbing alloys was mainly discussed on the basis of the bond orders between hydrogen and its first neighboring metals and the ionicity of the atom by means of the Muelliken population analysis. The bond orders between the hydrogen and the first neighboring metals in LaNi5H0.2 were larger than those in LaNi4.5Co0.5H0.2. The weak positive charge of H in LaNi5H0.2 interacted attractively with the weak negative charge of first neighboring Ni(1), however the charge of H interacted repulsively with the weak positive charge of the substituted first neighboring Co(1) in LaNi4.5Co0.5H0.2. Therefore, the hydrogen in LaNi5H0.2 interacted somewhat strongly with the first neighboring metals compared with that in LaNi4.5Co0.5H0.2. The bond orders between the hydrogen and the first neighboring atoms in LaNi5H0.2 were similar to those in La0.6Ce0.4Ni5H0.2, but the electrostatic interaction between H and Ni(1) of La0.6Ce0.4Ni5H0.2 was weakened compared with that of LaNi5H0.2. So, the hydrogen in LaNi5H0.2 interacted more strongly with the neighboring Ni(1) than that in LaNi4.5Co0.5H0.2. These findings were consistent with the thermodynamic experimental results.