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We investigated the hydrogen storage properties in a HoNi5-H system combined with TbNi5-H and DyNi5-H systems. Pressurecomposition (P-C) isotherms in the HoNi5-H system show one reversible pressure plateau during hydrogen absorption and desorption with a hysteretic phase transition. The maximum hydrogen storage capacity was found to be H/HoNi5 ¼ ca. 2, and no second plateau similar to that observed in other RNi5-H (R: rare earth) systems was observed, even at 196 K. Two pressure plateaux found in light rare earth-based RLNi5-H (RL ¼ La, Pr, Nd, Sm and Gd) systems tend to disappear in TbNi5-H and DyNi5-H systems and are scarcely present in HoNi5-H system. On the other hand, a novel pressure plateau appears at low hydrogen content in these systems due to the presence of a new hydride phase, and the plateau region extends from TbNi5-H and DyNi5-H to HoNi5-H systems. Contrary to the other plateaux, the pressures of the novel plateau during both hydrogen absorption and desorption decrease with decreasing unit cell volume of RNi5 compounds or with increasing atomic number of the R element in RNi5-H systems. To clarify the effect of 4f electrons in the R element in RNi5-H systems on the novel plateau, we have evaluated the hydrogen storage properties in an YNi5-H system. Our results show that YNi5 compound with lattice constants similar to those of TbNi5 and DyNi5 compounds has a similar P-C isotherm, suggesting that 4f electrons has no direct influence on the appearance of the novel plateau.

Appearance of a Novel Pressure Plateau in RNi<SUB>5</SUB>-H (R = Rare Earth) Systems