Evidence of Intermediate Hydrogen States in the Formation of a Complex Hydride

The Crystal Structures in Hydrogen Absorption Reactions of REMgNi4-Based Alloys (RE: Rare-Earth Metals)

REMgNi4-based alloys, RE(2−x)MgxNi4 (RE: rare-earth metals; 0 < x < 2), with a AuBe5-type crystal structure, exhibit reversible hydrogen absorption and desorption reactions, which are known as hydrogen storage properties. These reactions involve formation of three hydride phases. The hydride formation pressures and hydrogen storage capacities are related to the radii of the RE(2−x)MgxNi4, which in turn are dependent on the radii and compositional ratios of the RE and Mg atoms. The crystal structures formed during hydrogen absorption reactions are the key to understanding the hydrogen storage properties of RE(2−x)MgxNi4. Therefore, in this review, we provide an overview of the crystal structures in the hydrogen absorption reactions focusing on RE(2−x)MgxNi4.

First-principles investigation of LaMg2Ni and its hydrides

Using first-principles density functional theory calculations, the electronic structures of LaMg₂Ni and its hydrides LaMg₂NiH_4.5 (intermediate phase) and LaMg₂NiH₇ (fully hydrogenated phase), as well as the H adsorption on LaMg₂Ni (100) surface were investigated. For comparision, the atomic bonding characteristics of Co- and Pd-doped LaMg₂Ni, LaMg₂NiH_4.5 and LaMg₂NiH₇ compounds were also studied. Our aim is to provide new insights into the hydrogenation of LaMg₂Ni. The results show that the metallic intermediate hydride LaMg₂NiH_4.5 with Ni–H covalent bonds may act as the precursor state from the host compound LaMg₂Ni to the full hydride LaMg₂NiH₇. Upon LaMg₂Ni hydrogenation, the suppression of Mg–Ni and Ni–H interactions as well as the formation of La-H bonds favors for LaMg₂Ni–H formation.

Hydrogen-Release Reaction of a Complex Transition Metal Hydride with Covalently Bound Hydrogen and Hydride Ions

The hydrogen-release reaction of a complex transition metal hydride, LaMg₂ NiH₇ , composed of La³⁺ , 2×Mg²⁺ , [NiH₄ ]^4- and 3×H^- , was studied by thermal analyses, powder X-ray, and neutron diffraction and inelastic neutron scattering. Upon heating, LaMg₂ NiH₇ released hydrogen at approximately 567 K and decomposed into LaH_2-3 and Mg₂ Ni. Before the reaction, covalently bound hydrogen (H^c °^v. ) in [NiH₄ ]^4- exhibited a larger atomic displacement than H^- , although a weakening of the chemical bonds around [NiH₄ ]^4- and H^- was observed. These results indicate the precursor phenomenon of a hydrogen-release reaction, wherein there is a large atomic displacement of H^c °^v. that induces the hydrogen-release reaction rather than H^- . As an isothermal reaction, LaMg₂ NiH₇ formed LaMg₂ NiH_2.4 at 503 K in vacuum for 48 h, and LaMg₂ NiH_2.4 reacted with hydrogen to reform LaMg₂ NiH₇ at 473 K under 1 MPa of H₂ gas pressure for 10 h. These results revealed that LaMg₂ NiH₇ exhibited partially reversible hydrogen-release and uptake reactions.