Preparation of yttrium hydride-based photochromic films by reactive magnetron sputtering

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_x, followed by a controlled reaction with air that causes incorporation of oxygen into the material. An in-situ study of the YH_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_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 to 5.4 Å during the reaction. With the lattice parameter in the oxygen free YH_x being equal to that reported earlier for YH₂, our findings suggest that the non-reacted YH_x in this study is in-fact YH₂.