Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect

In contrast to the binary switchable mirror films $({\mathrm{YH}}_x,$ ${\mathrm{LaH}}_x,$ ${\mathrm{REH}}_x$ with RE:rare earth) which have a weak red transparency window in their metallic dihydride phase, rare-earth alloys containing magnesium are remarkable for the large contrast between their metallic dihydride and transparent trihydride phase. By means of structural, optical transmittance, and electrical resistivity measurements on a series of ${\mathrm{Y}}_{\text{1−y}}{\mathrm{Mg}}_y{\mathrm{H}}_x,$ films we show that this is due to a disproportionation of the alloy. While the yttrium dihydride phase is formed, Mg separates out, remaining in its metallic state. Upon further loading, insulating ${\mathrm{MgH}}_{\mathrm{2}}$ is formed together with cubic ${\mathrm{YH}}_{\text{3−δ}}.$ In this way Mg acts essentially as a microscopic optical shutter, enhancing the reflectivity of these switchable mirrors in the metallic state and increasing the optical gap in the transparent state.