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

本文用DV-X_2团簇方法研究了含有多种合金元素M的LaNi_5和Mg_2Ni的电子结构<alpha>。结果表明，在纯LaNi_5、纯Mg_2和纯Mg_2Ni中，尽管La和Mg原子对氢的亲合力比Ni原子大，但氢原子与Ni原子形成较强的化学键，而与La和Mg原子形成较强的化学键。这种金属-氢相互作用的存在是贮氢合金的特征，也表明贮氢合金中形成的金属间化合物的稳定性是根据贮氢的小金属多面体中原子之间的化学键的性质以及氢化过程中晶体结构的演变来理解的。例如，对于LaNi_5体系，La（M）-Ni键级与Ni（M）-Ni键级的比值与氢平衡平台压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。La、Zr、Ti、Mg）和非形成元素B（例如，Ni，Mn，Fe）的含量。此外，它表明，储氢合金的最佳A/B组成比是可预测的一个简单的参数，2 Bo（A-B）/ [Bo（A-A）+ Bo（B-B）]，其中Bo（A-B），Bo（A-A）和Bo（B-B）是在括号中给出的原子之间的键级。Mg_2Ni中的键级比最高，因为Bo（Mg-Ni）大于Bo（Mg-Mg）。因此，从这项研究中获得了一些合金设计的标准。

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.