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

Rare earth oxyhydrides REO x H(3-2x), with RE = Y, Sc, or Gdand a cationic FCC lattice,are reversibly photochromic in nature. It is known that structuraldetails and anion (O2-:H-) compositiondictate the efficiency of the photochromic behavior. The mechanismbehind the photochromism is, however, not yet understood. In thisstudy, we use H-1, H-2, O-17, and Y-89 solid-state NMR spectroscopy and density functional theory(DFT) calculations to study the various yttrium, hydrogen, and oxygenlocal environments, anion oxidation states, and hydride ion dynamics.DFT models of YO x H(3-2x) with both anion-ordered and anion-disordered sublatticesare constructed for a range of compositions and show a good correlationwith the experimental NMR parameters. Two-dimensional O-17-H-1 and Y-89-H-1 NMRcorrelation experiments reveal heterogeneities in the samples, whichappear to consist of hydride-rich (x & AP; 0.25)and hydride-poor domains (x & AP; 1) rather thana single composition with homogeneous anion mixing. The compositionalvariation (as indicated by the different x valuesin YO x H(3-2x)) is determined by comparing static H-1 NMR linewidths with calculated H-1-H-1 dipolarcouplings of yttrium oxyhydride models. The 1D O-17 MASspectrum demonstrates the presence of a small percentage of hydroxide(OH-) ions. DFT modeling indicates a reaction betweenthe protons of hydroxides and hydrides to form molecular hydrogen(H+ + H- & RARR; H-2). H-1 MAS NMR indicates the presence of a mobile component that,based on this finding, is attributed to trapped molecular H-2 in the lattice.