Structural and optical properties of Mg_yNi_1-yH_x gradient thin films in relation to the as-deposited metallic state

Thin Mg_yNi_1-yH_x films with a gradient in chemical composition are investigated by optical spectrophotometry, dc resistivity and X-ray diffraction measurements before and after exposure to hydrogen. In the metallic state crystalline Mg₂Ni is present for 0.6 ≤ y ≤ 0.8 and coexists with amorphous Mg and/or Ni. The hydride state is mainly characterized by the presence of Mg₂NiH₄ around the stoichiometric [Mg]/[Ni] = 2 composition and some MgH₂ on the Mg-rich side. The abrupt microstructural changes found in the as-deposited metallic state around the Mg-Mg₂Ni eutectic point correlate well with the compositional dependence of the optical properties in the hydride state. We conclude that the formation of the hydride depends directly on the detailed nature of the metallic parent phase. Furthermore, we demonstrate that high-throughput compositional screening via fiber-optic spectrophotometry is useful for hydride identification. When no structural long-range order is present, this provides a new tool for the search for hydrogen storage materials.