Exploring Multi-Anion Chemistry in Yttrium Oxyhydrides: Solid-State NMR Studies and DFT Calculations.

Rare earth oxyhydrides REO x H (3-2 x ) , 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 (3-2 x ) 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 (3-2 x ) ) 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.