Mg_y Ni (2 ≤ y ≤ 10) thin films covered with a Pd cap layer are hydrogenated in 1 0⁵ Pa H₂ between room temperature and 80 ° C and their dielectric function over(ε, ̃) is determined from reflection and transmission measurements. The hydrogenated Mg_y NiH_x thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg₂ NiH₄ host takes place at least for y ≤ 3.5 while no signature of MgH₂ is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg₂ NiH₄ (energy gap E_g = 1.6 eV) and MgH₂ (E_g = 5.6 eV) is expected.

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The structural, optical and dc electrical properties of Mg _xAl_1-x (0.2 ≤ x ≤ 0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the Mg_xAl_1-x layer undergoes a metal-to-semiconductor transition and MgH₂ is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH₂ and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction ¹H(¹⁵N, αγ)¹²C. Besides MgH₂, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x = 0.33). This suggests that the film is partially transformed into Mg(AlH₄)₂ at around this composition.

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The optical properties of Mg-TM (TM: Ni, Co, Fe; Mg:TM ≈ 2) thin films during hydrogenation are investigated using reflection and transmission measurements. Mg-Ni and Mg-Co show an unusual behavior upon exposure to hydrogen. The nucleation of the hydride starts at the film substrate interface and not - as intuitively expected - at the Pd covered top side of the film. Consequently, a double layer structure is formed. Reflection measurements are used to identify the loading behavior as the double layering yields to an remarkable optical black state at intermediate hydrogen concentrations. Photometric measurements on samples loaded at H2-pressures of 10 5 Pa show that in Mg-NiHx and Mg-CoHx the complex hydrides Mg2NiH4 and Mg2CoH5 are formed, whereas in Mg-Fex the formation of MgH2 is prevalent.@en

The peculiar optical patterns observed in Mg2NiHx films during hydrogenation with their applicability as selective tunable absorbers were discussed. These films showed large change in reflection that yielded very low reflectance at different hydrogen contents. It was shown that these films are suitable as smart coatings due to their energy tunability, high reflectance contrast, and switching capabilities. It was observed that these films can be used as a switchable solar absorber in solar collectors as the film in this state emits only 16% of the 100°C blackbody radiation. It was found that.@en

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.

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