  The effects of Co addition on the chemical and electronic structure of
PbTiO$_3$ were explored both by theory and through experiment. Cobalt was
incorporated to PbTiO$_3$ during sol gel process. The XRD data of the compounds
confirmed the perovskite structure for the pure samples. The XRD lines
broadened and showed emerging cubic-like features as the Co incorporation
increased. The changes in the XRD pattern were interpreted as double perovskite
structure formation. $^{207}$Pb NMR measurements revealed a growing isotropic
component in the presence of Co. In line with the experiments, DFT calculated
chemical-shift values corroborate isotropic coordination of Pb suggesting the
formation of cubic Pb$_2$CoTiO$_6$ domains in the prepared samples. The
state-of-the-art hybrid functional first-principles calculations indicate
formation of Pb$_2$CoTiO$_6$ with cubic structure and confirms that Co addition
can decrease oxygen binding energy significantly. Experimental UV-Vis
spectroscopy results indicate that upon addition of Co, the band gap is shifted
towards visible wavelengths which was confirmed by the energy bands and
absorption spectra calculations. The oxygen binding energies were determined by
temperature programmed reduction (TPR) measurements. Upon addition of Co, TPR
lines shifted to lower temperatures and new features appeared in the TPR
patterns. This shift was interpreted as weakening of oxygen cobalt bond
strength. The change in the electronic structure by the alterations of oxygen
vacancy formation energy and bond lengths upon Co insertion are determined by
DFT calculations.
