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

Abstract

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 μm. 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 1H and 89Y experiments are performed to obtain detailed information in polycrystalline samples and flexible thin films. In particular, the 89Y 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 present NMR observations may give some hints toward such a model.