Double layer formation in Mg–TM switchable mirrors (TM: Ni, Co, Fe)

Abstract 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 H 2 -pressures of 10 5  Pa show that in Mg–NiH x and Mg–CoH x the complex hydrides Mg 2 NiH 4 and Mg 2 CoH 5 are formed, whereas in Mg–Fe x the formation of MgH 2 is prevalent.