The photochromic effect was also observed in oxygen-containing gadolinium, dysprosium as well as erbium hydride and it has been concluded that reactively sputtered lanthanide-based hydrides behave similarly as the yttriumbased thin films, suggesting a common physical mechanism of the photochromic effect. [3] Based on charge neutrality considerations, the authors suggested a single-phase structure and introduced the term "metal oxyhydrides," [3] whereas previously such photochromic thin films were referred to as oxygen-containing metal hydrides. [1,2,4-7] The single-phase structure notion has been emphasized further by the proposal of a generalized simplified model of an aniondisordered fcc lattice. [8] This model was based on ion beam analysis and X-ray diffraction data, obtained from photo chromic oxygen-containing scandium, yttrium, and gadolinium hydride thin films. [8] Recently, diffraction data from photochromic oxygen-containing yttrium hydride were interpreted as indication for a multiphase nature. [9] Despite the fact that the phase formation has not been identified, [9] the interpretation of a multiphase nature is obviously in conflict with the notion of singlephase photochromic metal oxyhydride thin films. [3,8] Here, we critically evaluate the phase formation of photochromic oxygencontaining gadolinium hydride thin films. The evidence of a dual-phase structure in combination with a significant compressive residual stress state provides a straightforward explanation for the photochromic effect. Consequently, the notion of single-phase photochromic metal oxyhydrides, discussed in literature, is flawed.
We study the dependence of the photochromic effect on environment and triggering light. We demonstrate that the first darkening/bleaching cycle of freshly grown films is accompanied by a release of weakly bound hydrogen, most likely present at the grain boundaries. For consecutive photochromic cycles, we do not find further exchange of material with the environment. Moreover, we report bleaching kinetics dependent on the gas environment after darkening with light of energies below the optical bandgap of the film. For darkening with photon energies above the bandgap of the film, we report a linear relation between the degree of darkening and bleaching relaxation time irrespective of gas environment.
Photochromic oxygen-containing yttrium-hydride thin films are synthesized by argonmagnetron sputtering on microscope slides. Some of them are encapsulated with a thin, transparent and non-photochromic diffusion-barrier layer of either Al2O3 or Si3N4. Ion beambased methods prove that these protective diffusion barriers are stable and free from pinholes, with thicknesses of only a few tens of nanometers. Optical spectrophotometry reveals that the photochromic response and relaxation time for bothprotected and unprotectedsamples are almost identical. Ageing effects in the unprotected films lead to degradation of the photochromic performance (self-delamination) while the photochromic response for the encapsulated films is stable. Our results show that the environment does not play a decisive role for the photochromic process and encapsulation of oxygen containing rare-earth hydride films with transparent and non-organic thin diffusion barrier layers provides long-time stability of the films, mandatory for applications as photochromic coatings on e.g., smart windows.
Oxygen‐containing rare‐earth metal hydride thin films exhibit photo‐darkening upon light exposure; and in article number 2000822, the mechanism of the photochromic effect is unravelled by Marcus Hans and co‐workers. By combining X‐ray diffraction, transmission electron microscopy, and atom probe tomography, it is demonstrated that oxide and hydride phases are formed with significant compressive residual state. The reversible change in transparency can be understood as photon‐induced hydrogen transfer between two phases, identical in nature to the photochromic effect in bulk yttrium hydride at high pressure. [Artwork by CLS, RWTH Aachen University.]
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