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Double Perovskite Structure Induced by Co Addition to PbTiO$_3$ : Insights from DFT and Experimental Solid State NMR Spectroscopy

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.