The band gap energy (Eg) (Table 1) calculations reveal a
substantial reduction in the band gap of Ce0.95MxPr0.05xO2
when compared to CeO2. Substitution of Mn4+/Si4+ into CeO2
lattice induces formation of intermediate energy levels in the
CeO2 semiconductor on either side of fermi level that cause a
change in the band gap. The formation of mixed oxide alters the
optical characteristics of the resultant material due to the
contribution of Mn3d and Si3p to the conduction band. In the
case of solid solution containing Mn4+/Si4+ and Ce4+ the
conduction band is formed by Ce4f and Mn3d/Si3p resulting in
the band gap widening (2.78/2.90 eV). As far as the effect of
counter ions is concerned, Si co-doped pigments give
practically the same spectra, with a blue shift in the absorption
edge resulting in bright reddish brown hue. A gradual shift of
CT band can be actually appreciated along with the increase in