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Thermodynamic and corrosion study of Sm$_{1-x}$Mg$_x$Ni$_y$ (y = 3.5 or 3.8) compounds forming reversible hydrides

AB5 compounds (A = rare earth, B = transition metal) have been widely studied as anodes for Ni-MH applications. However, they have reached their technical limitations and the search for new promising materials with high capacity is foreseen. ABy compounds (2 < y < 5) are good candidates. They are made by stacking [AB5] and [A2B4] units along the c crystallographic axis. The latter unit allows a large increase in capacity, while the [AB5] unit provides good cycling stability. Consequently, the AB3.8 composition (i.e. A5B19 with three [AB5] for one [A2B4]) is expected to exhibit better cycling stability than the AB3.5 (i.e. A2B7 with two [AB5] for one [A2B4]). Furthermore, substitution of rare earth by light magnesium improves both the capacity and cycling stability. In this paper, we compare the hydrogenation and corrosion properties of two binary compounds SmNi$_{3.5}$ and SmNi$_{3.8}$ and two pseudo-binary ones (Sm,Mg)Ni$_{3.5}$ and (Sm,Mg)Ni$_{3.8}$. A better solid-gas cycling stability is highlighted for the binary SmNi$_{3.8}$. The pseudo-binary compounds also exhibit higher cycling stability than the binary ones. Furthermore, their resistance to corrosion was investigated.