Visualization of hydrogen migration in solids using switchable mirrors

Switchable mirrors(1-3) made of thin films of the hydrides of yttrium (YHx), lanthanum (LaHx) or rare-earth metals exhibit spectacular changes in their optical properties as xis varied from 0 to 3. For example, alpha-YHx<0.23 is a shiny, hexagonally close-packed metal, beta-YH2+/-delta is a face-centred cubic metal with a blue tint in reflection and a small transparency window at red wavelengths, whereas hexagonally close-packed gamma-YHx>2.85 is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of realtime visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium films in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission profiles along the length of the film vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled We can also induce electromigration of hydrogen, which diffuses towards the anode when a current flows through the film. Consequently hydrogen in insulating YH3-delta behaves as a negative ion, in agreement with recent strong-electron-correlation theories(4,5). This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.