Rare-earth oxyhydride REOxH3-2x 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.

Thin films of rare earth metal oxyhydrides show a photochromic effect, the precise mechanism of which is yet unknown. Here, we made thin films of NdH3-2xOx and show that we can change the band gap, crystal structure, and photochromic contrast by tuning the composition (O2-:H-) via the sputtering deposition pressure. To protect these films from rapid oxidation, we add a thin ALD coating of Al2O3, which increases the lifetime of the films from 1 day to several months. Encapsulation of the films also influences photochromic bleaching, changing the time dependency from first-order kinetics. As well, the partial annealing which occurs during the ALD process results in a dramatically slower bleaching speed, revealing the importance of defects for the reversibility (bleaching speed) of photochromism.

Doppler broadening positron annihilation spectroscopy depth profiles were collected on photochromic YO_xH_y thin films. In situ UV illumination of photochromic semiconductor YO_xH_y films leads to an increase in S-parameter and a large reduction in W-parameter, possibly caused by a change in the charge state of vacancies or the growth of hydrogen-rich metallic Y(O_x)H_y clusters, albeit that vacancy formation or changes in positronium formation during illumination might also play a role. Intriguingly, both the S- and W-parameters increase during thermal bleaching, indicating that another process takes place. The Doppler parameters do not return to their initial values after complete thermal bleaching, suggesting that persistent local rearrangements of vacancies and possibly hydride ions have occurred during the full photodarkening-thermal bleaching cycle. Positron annihilation lifetime spectroscopy shows that a small fraction of positronium is formed in as-deposited YO_xH_y films, indicating that the films contain some nanopores.

Rare-earth (RE) oxyhydride thin films prepared by reactive magnetron sputtering followed by air-oxidation show a color-neutral photochromic effect at ambient conditions within a wide composition range (REOxH3-2x where 0.5 ≤ x < 1.5). Due to the high degree of anion sublattice disorder present in these thin films, the structure models proposed for the related bulk materials are not directly applicable. Instead we use a combination of EXAFS analysis and lattice energy calculations to establish a fcc-based model linking the oxyhydrides to the related binary compounds. The oxide anions are found to occupy predominantly the tetrahedral sites in the fcc structure, which is attributed to electrostatic lattice energy minimization.

Thin films of yttrium oxy-hydride have interesting, reversible photochromic properties, the origin of which is poorly understood. To investigate the role of point defect mobility, we probed the effect of lattice contraction as induced by the addition of zirconium. Interestingly, we find no loss of photochromic contrast for Y _1-z Zr _z O _x H _y films with a small Zr cationic fractions (z < 0.15). At larger fractions the photochromic contrast is reduced. Zirconium is found to slow down the bleaching process, which suggests that the mobility of point defects may play a role in the thermal bleaching process. However, we cannot rule out substitution of zirconium in the YO _x H _y lattice which may also affect the photochromic properties.

Thin films of rare-earth (RE)-oxygen-hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE-O-H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REO _x H _3-2x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O ^2- and H ^- anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications.

In order to investigate the mechanism of the photochromic effect in yttrium oxy-hydride (YO_xH_y) thin films, Doppler broadening positron annihilation spectroscopy (PAS) was applied to probe the electronic structure and the presence of vacancies in YO_xH_y and related materials as a function of composition, UV illumination and thermal annealing. The Doppler S and W parameter depth profiles of a series of Y, yttrium di-hydride YH_1.9+δ and Y₂O₃ thin films show strong systematic changes caused by the distinct differences in electronic structure of the metals Y, YH_1.9+δ and the wide band gap insulator Y₂O₃. The Doppler broadening parameters of photochromic YO_xH_y (a semiconductor with a band gap of ~2.6eV) are intermediate to those of YH_1.9+δ and Y₂O₃. In order to probe the nanostructural changes related to the photochromic effect, the S parameter of YO_xH_y was monitored during in-situ UV illumination. A small but systematic increase of the S parameter was observed, possibly induced by generation of cation mono-vacancies or small vacancy clusters involving generated anion vacancies. The changes did not relax during bleaching under dark conditions, showing that the structural changes are not directly responsible for the photochromic mechanism. For temperatures above around 90°C, thermal annealing leads to a substantial increase in the Doppler S parameter, pointing to the formation of vacancies by local removal of hydrogen. Simultaneously, the optical band gap increases, consistent with an increase in O:H ratios.

Recently, thin films of yttrium oxy-hydride (YO_xH_y) were reported to show an unusual color-neutral photochromic effect promising for application in smart windows. Our present work demonstrates that also oxy-hydrides based on Gd, Dy, and Er have photochromic properties and crystal structures similar to YO_xH_y. Compared to YO_xH_y, the optical bandgaps of the lanthanide based oxy-hydrides are smaller while photochromic contrast and kinetics show large variation among different cations. Based on these findings, we propose that cation alloying is a viable pathway to tailor the photochromic properties of oxy-hydride materials. Furthermore, we predict that the oxy-hydrides of the other lanthanides are also potentially photochromic.