Impact of hydrogen concentration on the local structure of yttrium in photochromic yttrium oxyhydride

We report a systematic study of the relationship between structure and photochromic activity in yttrium oxyhydride (YHO) thin films deposited by DC magnetron sputtering under different ratios of ionization gas (Ar) and reactant gas (H 2 ) flows. The best photochromic performance was observed for intermediate ratios Ar/H 2 ≈ 9-12. Decreasing the Ar/H 2 ratio to about 5 promotes film growth with a (200) texture and suppresses photochromic activity. On the contrary, increasing the Ar/H 2 ratio to about 19 results in a dark-colored film without photochromic activity, exhibiting a metallic-like environment similar to Y foil or yttrium hydride. The local atomic structure and oxidation state of yttrium cations were probed using Y K-edge X-ray absorption spectroscopy. No shift in the absorption edge position was found under different deposition and illumination conditions, indicating an average Y 2.5+ oxidation state in YHO films. Analysis of extended X-ray absorption fine structure confirms that, while yttrium ions are on average arranged on a face-centred cubic lattice, local structural distortions induce a splitting of the second coordination shell of yttrium, well visible in both transparent and dark film states. These findings suggest that photochromism in YHO is not driven by changes in the average electronic structure of yttrium ions. The previously reported reduction of the yttrium charge may instead result from light-induced anion vacancy formation and defect migration. • No shift in the Y K-edge was observed under various deposition and illumination conditions, showing Y 2.5+ in YHO films. • Y K-edge EXAFS shows Y ions mainly on an FCC lattice, with local distortions splitting the second Y coordination shell. • XRD reveals that decreasing the Ar/H 2 ratio during sputtering promotes textured film growth and suppresses photochromism. • XAS shows Y oxidation unchanged and 2nd shell splitting, suggesting YHO photochromism arises from light-induced anion defects.