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When exposed to air, opaque yttrium dihydride YH₂ turns into transparent and photochromic yttrium oxyhydride YHO. YHO darkens reversibly when illuminated with light of adequate energy and intensity. The darkening is produced by light-induced oxygen release and diffusion. Therefore, the complete bleaching of the darkened films, and hence the reversibility of the process, only takes place if a source of oxygen is provided, e.g. in air.
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Baba et al. (2020) studied this question.
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