Metallic Intermediate Hydride Phase of LaMg₂Ni with Ni-H Covalent Bonding: Precursor State for Complex Hydride Formation

An intermediate phase LaMg₂NiH_4.6 found for the hydrogenation reaction of the ternary intermetallic compound LaMg₂Ni to the complex transition-metal hydride LaMg₂NiH₇ has been investigated experimentally and theoretically. It is known that this reaction induces only minor rearrangement of the host metal atoms, and this feature is also observed for the intermediate compound. Another important feature is that the electronic structure of the intermediate phase is metallic, which is free from the energy barrier associated with the metal-insulator transition. The hydrogenation reaction to the intermediate phase proceeds even at room temperature, when the Ni-H clusters are formed. The internal bonding of the Ni-H clusters has a covalent nature, the same as those of NiH₄ complexes in LaMg₂NiH₇. They most likely play as the precursor states for the following complex hydride formation. This picture is insightful to enhance the formation of complex hydrides.