Rare-earth oxyhydride REO_xH_3–2x thin films prepared by air-oxidation of reactively sputtered REH₂ dihydrides show a color-neutral, reversible photochromic effect at ambient conditions. The present work shows that the O/H anion ratio, as well as the choice of the cation, allow to largely tune the extent of the optical change and its speed. The bleaching time, in particular, can be reduced by an order of magnitude by increasing the O/H ratio, indirectly defined by the deposition pressure of the parent REH₂. The influence of the cation (RE = Sc, Y, Gd) under comparable deposition conditions is discussed. Our data suggest that REs of a larger ionic radius form oxyhydrides with a larger optical contrast and faster bleaching speed, hinting to a dependency of the photochromic mechanism on the anion site-hopping.

We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and ${\text{Mg}}_2\text{Ni}$ films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the $\sim 5\text{nm}$ thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that ${\text{Mg}}_2\text{Si}$ formation is fast relative to ${\text{MgH}}_2$ . The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to ${\text{Mg}}_2\text{Si}$ for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of ${\text{Mg}}_2\text{Si}$ after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of ${\text{Mg}}_2\text{Si}$ is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed ${\text{Mg}}_2\text{Si}$ .

Coimplantation of Zn and O ions into a single crystalline MgO and subsequent thermal annealing were applied in the synthesis of ZnO nanocrystals. Electron microscopy showed that rocksalt instead of wurtzite ZnO stabilizes for relatively large nanocrystals up to $\sim 15\mathrm{nm}$ , resulting from its small lattice mismatch with MgO of $\sim 1.7\%$ . The vacancies initially created by implantation induce favorable nanocrystal growth kinetics and are effectively absorbed during the nucleation and growth processes. The optical band edge of the ZnO nanocrystals was detected at $\sim 2.8\mathrm{eV}$ .

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.

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

We obtained evidence for the partial chemical segregation of as-deposited and hydrogenated ${\text{Mg}}_{1-\text{y}}{\text{Ti}}_{\text{y}}$ films $\left(0\le \text{y}\le 0.30\right)$ into nanoscale Ti and Mg domains using positron Doppler-broadening. We exclusively monitor the hydrogenation of Mg domains, owing to the large difference in positron affinity for Mg and Ti. The electron momentum distribution broadens significantly upon transformation to the ${\text{MgH}}_2$ phase over the whole compositional range. This reveals the similarity of the metal-insulator transition for rutile and fluorite ${\text{MgH}}_2$ . Positron lifetime studies show the presence of divacancies in the as-deposited and hydrogenated Mg-Ti metal films. In conjunction with the relatively large local lattice relaxations we deduce to be present in fluorite ${\text{MgH}}_2$ , these may be responsible for the fast hydrogen sorption kinetics in this ${\text{MgH}}_2$ phase.