Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. The structural, optical, and electrical transformations induced by hydrogen absorption and/or desorption in Mg-Ti thin films prepared by co-sputtering of Mg and Ti are investigated. Highly reflective in the metallic state, the films become highly absorbing upon H absorption. The reflector-to-absorber transition is fast, robust, and reversible over many cycles. Such a highly absorbing state hints at the coexistence of a metallic and a semiconducting phase. It is, however, not simply a composite material consisting of independent MgH 2 and TiH 2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The average structure resembles rutile MgH 2 at high Mg content and is fluorite otherwise. Of crucial importance in preserving the reversibility and the coherence of the system upon hydrogen cycling is the accidental equality of the molar volume of Mg and TiH 2 . The present results point toward a rich and unexpected chemistry of Mg-Ti-H compounds.

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Mg-Ti-H thin films are found to have very attractive optical properties: they absorb 87% of the solar radiation in the hydrogenated state and only 32% in the metallic state. Furthermore, in the absorbing state Mg-Ti-H has a low emissivity; at 400 K only 10% of blackbody radiation is emitted. The transition between both optical states is fast, robust, and reversible. The sum of these properties highlights the applicability of such materials as switchable smart coatings in solar collectors. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2212287͔ Many metal hydrides behave as switchable mirrors ͑i.e., their optical properties switch from reflective in the metallic state to transparent in the hydrogenated state͒. [1][2][3][4] In addition, magnesium-rare-earth ͑Mg-RE͒ and magnesium-transitionmetal ͑Mg-TM͒ switchable mirrors also exhibit an intermediate highly absorbing optical state on hydrogenation. The possibility to switch a film from a reflective state to an absorbing state suggests that such materials might be interesting for smart solar collectors, which absorb light in normal operation condition and switch to a reflective state to avoid overheating. Limiting the stagnation temperature of the solar collector makes it possible to use cheap materials such as plastics ͑generally not designed for high temperatures͒.In this letter we show that Mg y Ti 1−y thin films prepared by dc magnetron co-sputtering of Mg and Ti at room temperature ͑on quartz and CaF 2 ͒ satisfy the following requirements for a smart solar coating: ͑i͒ high absorption in the solar regime ͑0.5Ͻប Ͻ 4 eV͒, ͑ii͒ low emissivity in the thermal regime ͑ប Ͻ 0.5 eV͒, and ͑iii͒ reversibility. Three compositions are studied in detail: y = 0.70, y = 0.80, and y = 0.90. Typical deposition rates are 2 Å / s for Mg ͑150 W rf power͒, 0.1-1 Å / s for Ti ͑25-160 W dc power͒, and 1.3 Å / s for Pd ͑50 W dc power͒. All the films are covered with a Pd layer ͑10-50 nm͒ to promote dissociation of H 2 and to prevent oxidation of the underlying film.Reflection ͑R͒ and transmission ͑T͒ spectra are measured simultaneously during hydrogenation ͑pressures up to 1 bar H 2 ͒ in a Perkin Elmer Lambda 900 diffraction grating spectrometer ͑0.495Ͻប Ͻ 6.51 eV͒ and a Bruker IFS 66 Fourier transform infrared spectrometer ͑0.2Ͻប Ͻ 1.1 eV͒. The R-T spectra are measured through the transparent substrate at near normal incidence of the incoming beam. Figures 1͑a͒ and 1͑b͒ show the reflection and transmission spectra measured for 200 nm Mg y Ti 1−y / 10 nm Pd films ͑y = 0.90, 0.80, and 0.70͒ in the as-prepared and hydrogenated states ͑in 1 bar H 2 at room temperature͒. In the metallic state ͓Fig. 1͑a͔͒ all the films have a relatively high and featureless reflection that decreases with increasing Ti content.After hydrogen absorption, the reflection is low for all compositions, whereas significant transmission is observed only for the y = 0.90 sample. The combination of low reflection and low transmission in the hydrogenated state ͑y = 0.80 and 0.70 samples͒ gives rise to a highly a...

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X-Ray diffraction and electrochemical (de)hydrogenation were performed in situ to monitor the symmetry of the unit cells of Mg y Ti 100Ày thin film alloys (70 # y # 90 at.% Mg) along the pressurecomposition isotherms at room temperature. The diffraction patterns show that the crystal structures of all as-deposited alloys have a hexagonal closed packed symmetry. Inserting hydrogen transforms the structure to a body centered tetragonal structure for Mg 90 Ti 10 , whereas the unit cells of Mg 70 Ti 30 and Mg 80 Ti 20 transform into a face centered cubic symmetry. The structural change of the hydrides along with the ability to rapidly (de)hydrogenate the films emphasize the influence of the symmetry of the host lattice on the hydrogen transport properties. The lattice spacings of the unit cell of Mg 90 Ti 10 H x as a function of hydrogen content do not change much in the phase transformation region, indicating that only the fractions of the phases change. Remarkably, the lattice spacings found for Mg 70 Ti 30 H x in the two phase coexistence region reveal that not only the fractions but also the Mg-to-Ti ratio of both phases continuously change. Evidently, there is a large spread in the thermodynamic stability of the available sites for hydrogen. Since the X-ray diffraction patterns rule out large scale segregation, the results imply a nanostructured alloy with Ti-poor and Ti-rich regions and illustrate that the Mg and Ti atoms in Mg 70 Ti 30 are not randomly distributed.