The phase formation of a photochromic Gd_0.31(H_0.55O_0.45)_0.69 thin film, grown by reactive magnetron sputtering, is critically evaluated. Oxygen is preferably incorporated into the underdense columnar grain boundaries, when the as‐deposited gadolinium hydride film is exposed to ambient conditions. Two phases, Gd₂O₃ and GdH₂, are formed with significant compressive residual stress of 5.9 ± 1.5 GPa. These findings, extracted from transmission electron microscopy, X‐ray diffraction and atom probe tomography, provide a straightforward explanation for the photochromic effect. The mechanism can be understood as photon‐induced hydrogen transfer between the two phases, identical in nature to the photochromic effect in bulk yttrium hydride at high pressure.

We study the dependence of the photochromic effect on environment and triggering light. We demonstrate that the first darkening/bleaching cycle of freshly grown films is accompanied by a release of weakly bound hydrogen, most likely present at the grain boundaries. For consecutive photochromic cycles, we do not find further exchange of material with the environment. Moreover, we report bleaching kinetics dependent on the gas environment after darkening with light of energies below the optical bandgap of the film. For darkening with photon energies above the bandgap of the film, we report a linear relation between the degree of darkening and bleaching relaxation time irrespective of gas environment.

We report preferential orientation control in photochromic gadolinium oxyhydride (GdHO) thin films deposited by a two-step process. Gadolinium hydride (GdH_2-x) films were grown by reactive magnetron sputtering, followed by oxidation in air. The preferential orientation, grain size, anion concentrations and photochromic response of the films were strongly dependent on the deposition pressure. The GdHO films showed a preferential orientation along the [100] direction and exhibited photochromism when synthesized at deposition pressures of up to 5.8 Pa. The photochromic contrast was larger than 20% when the films were deposited below 2.8 Pa with a 0.22 H₂/Ar flow ratio. We argue that the relation of preferential orientation and the post deposition oxidation since oxygen concentration is known to be a key parameter for photochromism in rare-earth oxyhydride thin films. The experimental observations described above were explained by the decrease of the grain size as a result of the increase of the deposition pressure of the sputtering gas, followed by a higher oxygen incorporation.

Understanding the mechanism behind the photochromic properties and light-induced structural and chemical changes in yttrium oxyhydride remains a challenge. This lack of knowledge limits our ability to address degradation, enhance photochromic performance, control color, improve durability, and fully realize the material’s applications. Here, first-principles calculations indicate that anion vacancies may be responsible for the photochromic effect and other key properties of yttrium oxyhydride. These vacancies form deep localized energy levels in the band gap. Sunlight absorption transfers electrons from the valence band to these defect levels, altering the charge state of the vacancies and triggering both the photochromic effect and lattice relaxation. This relaxation involves yttrium cations shifting outward for positively charged vacancies and inward for negatively charged ones, thereby affecting the local environment around the yttrium cations by altering the second coordination shell. Using extended X-ray absorption fine structure spectroscopy on transparent and photo-darkened yttrium oxyhydride films, we show that UV light induces lattice relaxation in the second coordination shell, in agreement with first-principles calculations.