Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading -: art. no. 165411

The structural, thermodynamic and optical properties of Mg2Ni thin films covered with Pd are investigated upon exposure to hydrogen. Similar to bulk, thin films of metallic Mg2Ni take up 4 hydrogen per formula unit and semiconducting transparent Mg2NiH4-delta is formed. The dielectric function epsilon of Mg2Ni and fully loaded Mg2NiH4-delta is determined from reflection and transmission measurements using a Drude-Lorentz parametrization. Besides the two normal optical states of a switchable mirror-metallic reflecting and semiconducting transparent-Mg2NiHx exhibit a third black state at intermediate hydrogen concentrations with low reflection and essentially zero transmission. This state originates from a subtle interplay of the optical properties of the constituent materials and a self-organized double layering of the film during loading. Mg2NiH4-delta preferentially nucleates at the film/substrate interface and not-as intuitively expected-close to the catalytic Pd capping layer. Using epsilon(Mg2)Ni and epsilon(Mg2)NiH(4) and this loading sequence, the optical response at all hydrogen concentrations can be described quantitatively. The uncommon hydrogen loading sequence is confirmed by x-ray diffraction and hydrogen profiling using the resonant nuclear reaction H-1(N-15,alphagamma)C-12. Pressure-composition isotherms suggest that the formation of Mg2NiH4-delta at the film/substrate interface is mainly due to locally enhanced kinetics.