Critical composition dependence of the hydrogenation of Mg2±δ Ni thin films

The hydrogenation of metallic Mg2 Ni films was shown to proceed via a self-organized double layering of transparent Mg2 NiH4 and metallic Mg2 NiH0.3. For stoichiometries departing from Mg2 Ni we conclude from optical reflection, transmission and electrical measurements that the hydrogenation process involves two competing effects: (i) the nucleation of the initial Mg2 NiH4 layer near the substrate and the ensuing growth of this layer and (ii) the slow random nucleation of the same phase within the remaining part of the film. Which process dominates depends critically on the stoichiometry of the parent metal alloy.