After hydrogen incorporation, the unit cell contains two
cages occupied by OH−, another two occupied by H− and the
remaining eight cages of the unit cell are empty. Hydrogen
annealing results in no apparent change in the optical and
electrical properties of the material. However, upon irradiation
two optical absorption bands are induced, giving rise to
a persistent color change from white to green, together with
a dramatic conductivity increase from 10−10 to 0.3 S/cm at
room temperature. The first report suggested that the carriers
originated from ultraviolet irradiation according to