A.T.M. van Gogh

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.

Yttrium and lanthanum hydride films with switchable optical properties

Abstract We discovered that yttrium-, lanthanum- and rare earth-hydride films exhibit remarkable optical properties near their metal–insulator transition: the dihydrides are metallic and shiny while the trihydrides are semiconducting and transparent. The transition between the shiny and transparent state is reversible and can simply be induced by changing the H 2 gas pressure or the voltage in an electrolytic cell. No deterioration of the films is detected if they are protected by a thin palladium caplayer. The optical switching is also observed in hydrides of alloys of Y and La. Several theoretical models leading to an insulating and transparent trihydride state are discussed.

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.

In Situ Resistivity Measurements and Optical Transmission and Reflection Spectroscopy of Electrochemically Loaded Switchable YHx Films

We describe an experimental method for in situ resistivity measurements during the electrochemical hydrogen loading of thin, switchable metal hydride films. Using an oxygen-free electrolyte we are able to measure the hydrogen concentration in the films quantitatively and to determine the pressure-composition isotherms and the hydrogen concentration dependence of the resistivity. Furthermore, the optical properties can be investigated by simultaneous transmission and reflection spectroscopy. The power of these in situ measurements is discussed on the basis of results obtained on YHx films; possibilities for additional experiments are briefly described

Performance enhancement of metal-hydride switchable mirrors using Pd/AlOx composite cap layers

A drastic improvement of the optical properties and lifetime of switchable mirrors is obtained by placing a thin AlOx buffer layer between the Pd cap layer and the optically active, rare earth layer. The buffer layer lowers the minimum necessary Pd thickness to ∼1 nm, resulting in a ≈20% increase of the maximum transmittance. The optimal Pd and Al layer thicknesses are determined for the YHx and LaHx system using a powerful combination of optical and matrix film techniques. The AlOx buffer is shown to be superior to the native oxide layers YOx and LaOx. The buffer layer is essential for lanthanum, which is a particularly vulnerable, but fundamentally very important material. Using this composite cap layer, we have been able to switch LaHx films several times.

Structural, optical and electronic properties of LaMgHx switchable mirrors

Structural, optical and electronic properties of lanthanum magnesium alloy thin films are studied in situ in real time during hydrogenation. X-ray data show that the as-deposited films contain the intermetallic phase LaMg with CsCl structure as well as fcc β-La and fcc LaHx. Hydrogenation initiates a transformation of β-La to β-LaHx, while the intermetallic phase disproportionates. The fcc structure stays intact during further uptake of hydrogen up to LaH3, and Mg transforms to the rutile structure of MgH2. During hydrogen absorption the alloys change from a good conducting, metallic state with high reflectance, to a transparent color neutral insulator with a band-gap of about 3 eV.

Highly reflecting Y/Mg-Hx multilayered switchable mirrors

Optical, structural and thermodynamic properties of Y/Mg–Hx multilayered switchable mirrors are investigated and compared with YMgHx-alloys and pure YHx. Multilayers clearly have superior reflectance in the low-hydrogen state over the whole range of photon energies, 0.72<hω<3.5 eV, investigated in this work. In the transparent high-hydrogen state the absorption edge of the multilayers with the same overall composition is shifted in energy on increasing the individual layer thicknesses. Thus, the absorption edge is non-trivially depending on the distribution of Mg in the film. Moreover, the optical contrast of multilayers is much higher than for pure YHx and their optical switching does not suffer from hysteretic effects. Finally, Mg and YH2 transform almost at the same time to MgH2 and YH3, respectively, with a heat of formation of −30.9 kJ/mol H in absorption and −36.6 kJ/mol H in desorption.

Dihydride transmission quenching in switchable La1−zYzHx mirrors

Dihydrides of La1−zYz films with 0<z<1 are studied by reflectance-transmittance measurements and ellipsometry in the visible and near infrared spectral range. The dihydride transmission window, present in LaH2 and YH2, is strongly suppressed in thin La1−zYzH2 films. By modelling the complex dielectric function of the alloys with a Drude–Lorentz parametrization we show that the suppression is due to atomic disorder, mainly via optical absorptions at energies larger than 4.5 eV.