Structural and optical properties of Mg₂NiH_x switchable mirrors upon hydrogen loading

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 Mg₂NiH_4-δ is formed. The dielectric function ε of Mg₂Ni and fully loaded Mg₂NiH_4-δ 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 - Mg₂NiH_x 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. Mg ₂NiH_4-δ preferentially nucleates at the film/substrate interface and not - as intuitively expected - close to the catalytic Pd capping layer. Using ε_Mg2Ni and εMg ₂NiH₄ 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 ¹H(¹⁵N, αγ)¹²C. Pressure-composition isotherms suggest that the formation of Mg₂NiH_4-δ at the film/substrate interface is mainly due to locally enhanced kinetics.