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Switchable mirrors 1±3 made of thin ®lms of the hydrides of yttrium (YH x ), lanthanum (LaH x ) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, a-YH x,0.23 is a shiny, hexagonally close-packed metal, b-YH 26d is a face-centred cubic metal with a blue tint in re¯ection and a small transparency window at red wavelengths, whereas hexagonally close-packed g-YH x.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 ®lms in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission pro®les along the length of the ®lm 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¯ows through the ®lm. Consequently, hydrogen in insulating YH 3-d 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.Diffusion of hydrogen in metals has attracted considerable attention 6±8

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