It has been recently demonstrated that yttrium oxyhydride (YHO) films can exhibit reversible photochromic properties when exposed to illumination at ambient conditions. This switchable optical property enables their utilization in many technological applications, such as smart windows, sensors, goggles, and medical devices. However, how the composition of the films affects their optical properties is not fully clear and therefore demands an investigation. In this paper, the composition of YHO films manufactured by reactive magnetron sputtering under different conditions is deduced in a ternary diagram from time-of-flight elastic recoil detection analysis. The results suggest that stable compounds are formed with a specific chemical formula—$\mathrm{Y}{\mathrm{H}}_{2\text{−}\delta }{\mathrm{O}}_{\delta }$. In addition, optical and electrical properties of the films are investigated, and a correlation with their compositions is established. The corresponding photochromic response is found in a specific oxygen concentration range ($0.45<\delta <1.5$) with maximum and minimum of magnitude on the lower and higher border, respectively.

When exposed to air, metallic yttrium dihydride ${\mathrm{YH}}_2$ films turn into insulating and transparent yttrium oxyhydride (YHO). The incorporation of oxygen causes the lattice expansion of ${\mathrm{YH}}_2$ and the emergence of photochromic properties, i.e., YHO darkens reversibly when illuminated with light of adequate energy and intensity. However, the adequate bleaching of the photodarkened samples once the illumination has stopped is much faster in air than in inert atmosphere. According to this experimental evidence, the photochromic mechanism has to be related to an oxygen diffusion and exchange process. Since this process is accompanied by a lattice expansion/contraction, it can be said that YHO “breathes” when subjected to illumination/darkness cycling. Another interesting side effect of the breathing is the unexpected enhancement of the hydrophobicity of the YHO samples under illumination. A theoretical model able to explain the breathing in YHO is presented, together with the discussion of other alternative explanations.

Yttrium oxyhydride (YHO) is a rare‐earth‐metal oxyhydride that has attracted considerable attention due to its outstanding photochromic properties. The transparency of YHO thin films across the infrared and visible spectral regions is reduced considerably under UV illumination (photodarkening) and recovers when the illumination is removed (bleaching). Although oxygen diffusion has been shown to be necessary for these processes, the exact mechanism for the photochromic behavior is not yet understood. In this work, infrared spectroscopy is utilized to investigate the effect of temperature on the photochromic properties of YHO thin films. The measurements show that YHO can photodarken at temperatures as low as 5 K, where anion diffusion is expected to be severely limited. The bleaching of the films is small, but not zero, for temperatures between 5 and 50 K. A stepwise recovery of the transmittance is observed as the temperature of the films is increased above 100 K up to 250 K. The temperature‐dependent data show that anion diffusion is not required to explain the photochromic behavior of YHO, and that an additional mechanism (or mechanisms), e.g., electronic charge transfer, contributes to the photochromic behavior of YHO, as well as other rare‐earth‐metal oxyhydrides.