Synthesis of photochromic oxygen-containing yttrium hydride
  Photochromic oxygen-containing yttrium hydride has been synthesized using a two step process. The process consists of an initial sputter deposition of oxygen-free yttrium hydride YH$_\mathrm{x}$, followed by a controlled reaction with air that causes incorporation of oxygen into the material. An in-situ study of the YH$_\mathrm{x}$ as it reacted with oxygen was made possible by applying an aluminium capping layer with a low but non-zero oxygen permeability prior to air exposure. During the reaction, the visual appearance of the YH$_\mathrm{x}$ went from dark and opaque to transparent and yellowish. This transition was accredited incorporation of oxygen into the material, as shown through analysis with energy dispersive x-ray spectroscopy (EDS) and x-ray photoemission spectroscopy (XPS). Grazing incidence x-ray diffraction (GIXRD) studies revealed an fcc structure both before and after oxygen exposure, with a lattice parameter that increased from 5.2 {\AA} to 5.4 {\AA} during the reaction. With the lattice parameter in the oxygen free YH$_\mathrm{x}$ being equal to that reported earlier for YH$_\mathrm{2}$, our findings suggest that the non-reacted YH$_\mathrm{x}$ in this study is in-fact YH$_\mathrm{2}$. 
J. Montero, F. A. Martinsen, M. Lelis, S. Zh. Karazhanov, B. C.   Hauback and E. S. Marstein
cond-mat.mtrl-sci