Highlighting the stability control of superlattice structures by fine tuning of subunit volumes

ABy compounds with superlattice structures are considered as new generation materials for hydrogen storage and very promising as negative electrode for Ni-MH batteries. These compounds can be considered as the stacking of two fundamental layers along the c crystallographic axis: [A2B4] and [AB5]. Binary RNix (R = rare earths) compounds adopting stacking structures absorb large quantity of hydrogen but show poor reversibility. The superlattice structures are built by constraining the ab planes of the [A2B4] subunit to that of the more stable [AB5] one. The volume mismatch between the two subunits is the main cause of the multi-plateau behavior of the binary RNix (R = rare earths) compounds. The multi-element compounds show often improved hydrogen storage properties. In this review we highlight the structural evolution with the element substitution and the correlation between the subunit volume matches and the hydrogenation properties.