A View on Formation Gap in Transition Metal Hydrides and Its Collapse

Late transition metals (LTMs) do not form hydrides under conventional experimental conditions except for palladium (Pd). The incorporation of a small amount of metal with low electronegativity converts LTMs into hydrogen (H) storage intermetallics (IMs) such as LaNi₅. We examined the critical property of the H absorbing ability of LTMs and found that the lattice softness of Pd is a unique parameter for H insertion. This idea is applicable to H storage IMs. Indeed, negatively charged LTM ions, such as Ni^δ- in LaNi₅, play a crucial role in hydrogenation. These ions cause lattice softening, which renders H solution possible. As a result, we propose that lattice softness can be an effective criterion for the development of new H storage IMs.