Characteristics of hydrogen storage alloys in view of molecular orbital method and design of magnesium-based alloys with high hydrogen capacity

采用DV-X_< α >簇法研究了含有多种合金元素M的LaNi_5和Mg_2Ni的电子结构。发现在纯LaNi_5、纯Mg_2或纯Mg_2Ni中，氢原子与Ni原子形成较强的化学键，而与La或Mg原子形成较强的化学键，尽管在二元金属-氢体系中，La或Mg原子对氢的亲和力大于Ni原子。这种金属-氢相互作用的存在是储氢合金的特征。研究还表明，储氢合金中氢化物的稳定性可以从储存氢的小金属多面体中原子间化学键的性质以及氢化过程中晶体结构的演变中得到很好的理解。例如，对于LaNi_5体系，La (M) -Ni键序与Ni (M) -Ni键序的比值与氢的平衡平台压力的实验数据有很好的相关性。另一方面，对于Mg_2Ni体系，Ni (M) -Mg键强度和Ni (M) -H键强度与氢化物的生成焓有很好的相关性。进一步阐明了氢化物形成元素A（如La、Zr、Ti、Mg）和非形成元素B（如Ni、Mn、Fe）的作用。此外，储氢合金的最佳A/B组成比可以用一个简单的参数来预测，2bo (A-B) / [Bo (A-A) + Bo (B-B)]，其中Bo (A-B), Bo （A-A）和Bo （B-B）是括号中给出的原子之间的键序。在储氢合金中，Mg_2Ni的键序比最高，因为Bo （Mg-Ni）比Bo （Mg-Mg）大。从而为合金的设计提供了一定的依据。

The electronic structures of LaNi_5 and Mg_2Ni both containing variety of alloying elements, M, are investigated by the DV-X_<alpha> cluster method. It is found that hydrogen atoms make a strong chemical bond with Ni atoms rather than La or Mg atoms in pure LaNi_5 or in pure Mg_2 or in pure Mg_2Ni, despite the larger affinity of La or Mg atoms for hydrogen than Ni atoms in the binary metal-hydrogen system. The presence of such metal-hydrogen interactions is characteristic of hydrogen storage alloys.It is also shown that the stability of hydrides formed in the hydrogen storage alloys is well understood in terms of the nature of the chemical bond between atoms in small metal polyhedra where hydrogen is stored and also of the crystal structural evolution during hydrogenation. For example, for the LaNi_5 system, the ratio of the La (M) -Ni bond order to the Ni (M) -Ni bond order correlates well with the experimental data of the equilibrium plateau pressure of hydrogen. On the other hand, for the Mg_2Ni system, both the Ni (M) -Mg bond strength and the Ni (M) -H bond strength correlate well with the measured enthalpy of formation for the hydride.Further, the roles of the hydride forming elements, A, (e.g., La, Zr, Ti, Mg) and non-forming elements, B, (e.g., Ni, Mn, Fe) are elucidated. Also, it is shown that the optimum A/B compositional ratio of hydrogen storage alloys is predictable in terms of a simple parameter, 2 Bo (A-B) / [Bo (A-A) + Bo (B-B) ] , where the Bo (A-B) , the Bo (A-A) and the Bo (B-B) are the bond orders between atoms given in the parentheses. This bond order ratio is highest in Mg_2Ni among the hydrogen storage alloys, because Bo (Mg-Ni) is larger than Bo (Mg-Mg) . Thus, some criteria for alloy design were obtained from this study.