Structural properties and anion dynamics of yttrium dihydride and photochromic oxyhydride thin films examined by <i>in situ</i> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>μ</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mi>SR</mml:mi></mml:mrow></mml:math>

Thin films of rare-earth metal oxyhydrides, such as yttrium oxyhydrides (${\mathrm{YH}}_{3\ensuremath{-}2x}{\mathrm{O}}_{x}$), show a photochromic effect where the transparency decreases reversibly upon exposure to UV light. However, exact mechanism behind this is unknown. In paper, we describe behavior ${\mathrm{YH}}_{3\ensuremath{-}2x}{\mathrm{O}}_{x}$ thin films, with different ${\mathrm{O}}^{2\ensuremath{-}}:{\mathrm{H}}^{\ensuremath{-}}$ ratios, under dark and illuminated conditions using in situ muon spin relaxation (${\ensuremath{\mu}}^{+}\mathrm{SR}$), compare that an oxygen-free reference compound, dihydride (${\mathrm{YH}}_{2\ensuremath{-}\ensuremath{\delta}}$). The acts local magnetic probe our compounds, giving information related electronic, structural, properties. Although ${\mathrm{YH}}_{2\ensuremath{-}\ensuremath{\delta}}$ parent compound ${\mathrm{YH}}_{3\ensuremath{-}2x}{\mathrm{O}}_{x}$, these two materials different---the electrostatically interacts primarily ${\mathrm{H}}^{\ensuremath{-}}$ (dihydride) or ${\mathrm{O}}^{2\ensuremath{-}}$ (oxyhydride)---leading use theoretical models. For ${\mathrm{YH}}_{2\ensuremath{-}\ensuremath{\delta}}$, observed formation entangled H-$\ensuremath{\mu}$ complex onset ${\mathrm{Mu}}^{+}$ diffusion rearrangement above 150 K (${E}_{A,\mathrm{\ensuremath{\Gamma}}}=67\ifmmode\pm\else\textpm\fi{}13\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$). adopted transition state model, ${\mathrm{Mu}}^{0}$ gradual recovery take place, accompanied by ${\mathrm{Mu}}^{+}\text{\ensuremath{-}}{\mathrm{O}}^{2\ensuremath{-}}$ polaron at Y cation. activation energy (${E}_{A,\mathrm{dia}}$) associated dependent on lattice lower for higher H content (${E}_{A,\mathrm{dia}}=29--45\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$). illumination further reduces barrier all measured suggesting involves reversible structural during photodarkening.