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, medical devices, etc. However, how the composition of the films affects their optical properties is not fully clear and therefore demands a straightforward investigation. In this work, the composition of YHO films manufactured by reactive magnetron sputtering under different conditions is deduced in a ternary diagram from Timeof-Flight Elastic Recoil Detection Analysis (ToF-ERDA). The results suggest that stable compounds are formed with a specific chemical formula-YH 2-δ O δ. 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 < δ < 1.5) with maximum and minimum of magnitude on the lower and higher border, respectively.

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When exposed to air, metallic yttrium dihydride YH 2 films turn into insulating and transparent yttrium oxyhydride (YHO). The incorporation of oxygen causes the lattice expansion of 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.