  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.
