Abstract
Rare earth oxyhydrides REO(x)H((3-2x)), with RE = Y, Sc, or Gd and a cationic FCC lattice, are reversibly photochromic in nature. It is known that structural details and anion (O(2-):H(-)) composition dictate the efficiency of the photochromic behavior. The mechanism behind the photochromism is, however, not yet understood. In this study, we use (1)H, (2)H, (17)O, and (89)Y solid-state NMR spectroscopy and density functional theory (DFT) calculations to study the various yttrium, hydrogen, and oxygen local environments, anion oxidation states, and hydride ion dynamics. DFT models of YO(x)H((3-2x)) with both anion-ordered and anion-disordered sublattices are constructed for a range of compositions and show a good correlation with the experimental NMR parameters. Two-dimensional (17)O-(1)H and (89)Y-(1)H NMR correlation experiments reveal heterogeneities in the samples, which appear to consist of hydride-rich (x ≈ 0.25) and hydride-poor domains (x ≈ 1) rather than a single composition with homogeneous anion mixing. The compositional variation (as indicated by the different x values in YO(x)H((3-2x))) is determined by comparing static (1)H NMR line widths with calculated (1)H-(1)H dipolar couplings of yttrium oxyhydride models. The 1D (17)O MAS spectrum demonstrates the presence of a small percentage of hydroxide (OH(-)) ions. DFT modeling indicates a reaction between the protons of hydroxides and hydrides to form molecular hydrogen (H(+) + H(-) → H(2)). (1)H MAS NMR indicates the presence of a mobile component that, based on this finding, is attributed to trapped molecular H(2) in the lattice.