Solid-State NMR Studies of the Photochromic Effects of Thin Films of Oxygen-Containing Yttrium Hydride

Oxygen containing yttrium hydride thin films show a unique reversible change in optical properties as a result of illumination. In this article, different solid state NMR methods are used to follow the changes during such a photochromic process. The novel stripline probe technology provides a high sensitivity for NMR experiments on thin film samples with a thickness of approximately 1 mu m. A key observation is that these photochromic films show spectral evidence of a highly mobile hydrogen species, which disappears upon illumination with white light. A relaxation back to the transparent phase is observed after a few days, accompanied by the reappearance of the mobile proton NMR signal. These results are compared with NMR experiments on pure yttrium hydride thin films. Magic-angle spinning H-1 and Y-89 experiments are performed to obtain detailed information in polycrystalline samples and flexible thin films. In particular, the Y-89 MAS experiments indicate changes in the local environment of the Y nucleus upon optical illumination. A detailed structural model for the photochromic change is not yet clear, but the presetn NMR observations may give some hints toward such a model.