Rare-Earth oxyhydrides (REH3-2xOx) are characterized by photodarkening when illuminated by photons having an energy exceeding that of the band gap. We propose that the film is segregated in hydrogen rich and hydrogen poor areas. Upon illumination, the excited electrons reduce the three-valent cations inducing an insulator to metal transition in the hydrogen rich entities. These small metallic oxyhydride clusters are responsible for the enhanced optical absorption. In the surrounding semiconductor matrix the photoexcitation induces a transition from p to n-type conductivity. This persistent photoconductivity is due to trapping of the holes by hydride ions. As a result, the Fermi level rises above the conduction band inducing a Burstein-Moss effect and a large increase in the conductivity.@en

Rare earth oxyhydrides REO_xH_(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 ¹H, ²H, ¹⁷O, and ⁸⁹Y 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_xH_(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 ¹⁷O-¹H and ⁸⁹Y-¹H 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_xH_(3-2x)) is determined by comparing static ¹H NMR line widths with calculated ¹H-¹H dipolar couplings of yttrium oxyhydride models. The 1D ¹⁷O 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^- → H₂). ¹H MAS NMR indicates the presence of a mobile component that, based on this finding, is attributed to trapped molecular H₂ in the lattice.

@en

In this paper, we investigate by ab initio DFT how the O:H ratio influences the formation and lattice energy, metastability, and optical properties of Y and La anion-disordered ROxH3-2x oxyhydrides. To achieve this, a set of special quasirandom structures (SQS) is introduced to model anion-disorder along the whole RH3-R2O3 composition line. A comparison with an extensive set of anion-ordered polymorphs of the same composition shows the comparable energy of the anion-disordered phase, which, in particular, in the H-rich composition interval showed the lowest relative energy. In turn, the metastability of the anion-disordered phase depends on the cation size (Y versus La), which determines the maximum H content above which the CaF2-type structure itself becomes unstable. To overcome the accuracy limitations of classical DFT, the modified Becke-Johnson (mBJ) scheme is employed in the study of the electronic properties. We show that major differences occur between H-rich and O-rich R oxyhydrides, as the octahedral H- present for x<1 form electronic states at the top of the valence band, which reduce the energy band gap and dominate the electronic transitions at lower energies, thus increasing the refractive index of the material in the VIS-nIR spectral range. Comparing the DFT results to experimental data on photochromic Y oxyhydride films reinforces the hypothesis of anion-disorder in the H-rich films (x<1), while it hints towards some degree of anion ordering in the O-rich ones (x>1). Our paper exemplifies a strategy to calculate ab initio the electronic/optical properties of a wide range of materials with occupational disorder.

@en