D.G. Nagengast

Visualization of hydrogen migration in solids using switchable mirrors

Switchable mirrors made of thin films of the hydrides of yttrium (YHx), lanthanum (LaHx) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, α-YHx <0.23 is a shiny, hexagonally close-packed metal, β-YH2±δ is a face-centred cubic metal with a blue tint in reflection and a small transparency window at red wavelengths, whereas hexagonally close-packed γ-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 real-time 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−δ behaves as a negative ion, in agreement with recent strong-electron-correlation theories,. This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.

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

In contrast to the binary switchable mirror films (YHx, LaHx, REHx 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 Y1−yMgyHx, 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 MgH2 is formed together with cubic YH3−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.

Epitaxial switchable yttrium-hydride mirrors

By means of x-ray scattering and scanning probe microscopy it is shown that high-quality epitaxial Y films can be deposited on (111)-CaF2 substrates. The films can reversibly be switched from metallic YH2 to transparent insulating YH3−δ. Although hydrogen absorption involves an expansion of the lattice and a symmetry change from hcp to fcc, the epitaxiality of the film remains intact during the switching process. The transparency and the insulating nature of the substrate opens unique possibilities to investigate electrically and optically these switchable mirror films in the single crystalline state.

Growth and hydrogenation of epitaxial yttrium switchable mirrors on CaF2

Rutherford backscattering (RBS) ion channeling measurements and X-ray diffraction experiments are performed to study the epitaxial nature of as-deposited yttrium on CaF2111 substrates and the effect of hydrogenation on the crystalline quality. The RBS and X-ray results clearly demonstrate the unique epitaxial relation between as-deposited films and the substrate, which is preserved upon loading with hydrogen. X-Ray diffraction reveals: (i) a remarkably large lattice expansion in the direction normal to the substrate, which decreases with increasing film thickness; and (ii) an in-plane compression of the lattice. This peculiar result is related to the difference in thermal expansion coefficients of film and substrate. RBS ion channeling measurements reveal a thickness dependence of the mismatch-induced stresses. As expected, the stresses relax with increasing distance from the film/substrate interface, but surprisingly, even with films as thick as 400 nm considerable dechanneling is still observed at the film surface. Film quality, i.e. the film/substrate mismatch as well as the induced stresses and their relaxation, are discussed in relation to atomic force microscopy (AFM) results on these epitaxial films.