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Light-induced breathing in photochromic yttrium oxy-hydrides

When exposed to air, metallic yttrium dihydride YH2films turn into insulating and transparent yttrium oxy-hydride (YHO). The incorporation of oxygen causes the lattice expansion of YH2 and the emergence of photochromic properties. However, the oxidation of YH2 is not completely irreversible: under illumination some oxygen atoms move towards the YHO surface, leaving behind an oxygen-deficient bulk (responsible for the photochromic darkening and observed lattice contraction). Under illumination, and according to experimental evidence, some oxygen atoms can effectively leave the film, being replaced by other oxygen atoms once the illumination has stopped, i.e., YHO "breathes" when subjected to illumination/darkness cycling. Based on this "breathing", YHO films become more hydrophobic under illumination conditions than when kept in darkness. Ab-initio calculations point to the light-induced weakening of the Y-O bond as the possible mechanism for explaining these experimental observations.