the coloring mechanism is based on the shift of the
charge transfer band of the semiconductor CeO2 to higher
wavelengths, introducing an additional electronic level by
doping praseodymium. The 4f valence shell of Ce4+([Xe]) in
cerianite is empty, and that of O2 ([Ne] 2s2 2p6) is full: adjacent
Ce4+ ions are virtually in contact in the fluorite lattice and, as a
result, 4f orbitals overlap in a cationic conduction band;
similarly, overlap of 2p orbitals of oxygen ions gives to an
anionic valence band. The Ce–O bonds have a stronger ionic
character, and hence a smaller contribution of the Ce 5d and 4f
orbitals in the VB, in CeO2 [25]. The band gap between the
anionic band and the cationic band is 3.01 eV. By doping CeO2
with Pr4+ ions, the 4f1 electron of the praseodymium valence