show increased absorbance and results in dark brown
hue. The width of the band gap is determined by the extent of
overlap of the valence orbitals and by the difference between
the electronegativities of the cations and anions involved [27–
29]. The electronegativities of the metals only vary in small
steps and the substitution of one metal by the other usually
produces small shifts in elctronegativity difference and hence
small change in bandgap is expected on co-substitution of Mn
or Si with Pr in the cerianite lattice. As Si has higher
electronegativity than Mn and Ce, the difference in electronegativity
between the cations and oxygen decreases and more
shift in absorption edge is expected for silicon co-doped
samples than that of manganese containing compounds. The
change in color of CP with increasing content of doped element
accounts for the shift in absorption edge, which is caused by the
change in chemical bonding nature by incorporating higher
electronegative metal ion into the praseodymium–cerium
matrix.