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 temperature dependent optical properties of certain thin films are used in so-called thermochromic devices. As an alternative to a single layer of, e.g. VO₂, which displays a temperature dependent optical transmission, we propose the use of an encapsulated metal-hydride bilayer (i.e. a switchable mirror and a hydrogen storage layer). Due to differences in the thermodynamic properties of the two metal-hydride layers, a temperature change results in the migration of hydrogen from one layer to the other, thereby changing the optical properties of the switchable mirror metal-hydride. Depending on the materials chosen the transition can be optimized for transmission, reflection or absorption. Hence, the use of metal-hydride bilayers allows the fabrication of a thermochromic absorber. The thermodynamical background and the first results of such an all-solid-state thermochromic device are presented.

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