Modification of the hydrogenation properties of LaNi5 upon Ni substitution by Rh, Ir, Pt or Au

Highlights: → Hydrogen absorption properties of LaNi5-xMx intermetallic compounds (M = Rh, Ir, Au). → Stability of LaNi5-xMx (M = Rh, Pt) intermetallics and hydrides using ab initio calculations. → Influence of the substituting elements (geometric, thermodynamic and electronic) on the hydride stability. - Abstract: The hydrogenation properties of the LaNi5-xMx (M = Rh, Ir, Au) compounds have been studied. The Ni substitution has several consequences: pressure plateau splitting and increase of plateau pressure. This latter observation disagrees with the general rule that a cell volume increase of the alloy should result in a plateau pressure lowering. In order to elucidate the origin of this anomalous behaviour, DFT calculations have been performed on both LaNi5-xRhx and LaNi5-xPtx intermetallic compounds, which, according to the present and previous experimental work, present a similar anomaly. We discuss our results in light of the models proposed in the literature. We conclude that, in the case of a Ni substitution by 4d or 5d elements, the size effect alone fails in predicting the hydrogen absorption properties while the rule of reverse stability is obeyed.