Molten salts synthesis and Raman, XPS, and UV–vis spectroscopy study of Zn-doped Y2Ti2O7- δ pyrochlore

In this study, a series of pyrochlore-type compounds (Y2-xZnx)Ti2O7- δ with x = 0.0, 0.1, and 0.2 were synthesized using the molten salt method. The samples were studied by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. Rietveld refinement of the XRD patterns shows that Zn2+ ions occupy Y3+ sites. Consequently, a lattice parameter and ionic radius ratio rA/rB decrease; consequently, the crystal structure is less ordered. Raman spectroscopy shows that the disorder manifests by broadening of FWHM Raman modes and increasing the A1g Raman mode frequency as the Zn content (x) rises. Another repercussion arising from the disorder observed in the pyrochlore is the occurrence of oxygen vacancies, which are associated with ionic conductivity and are localized at 531 eV in the high-resolution O1s XPS spectra.First-principles studies using LDA + U show that Y2Ti2O7 has a 3.07 eV electronic band gap, slightly lower than the 3.12 eV experimentally reported. The valence band (VB) is a contribution of O 2p hybridized states with Ti-3d and Y-4d, while the conduction band (CB) has Ti-3d states hybridized with O-2p states. The VB XPS spectrum and calculated DOS for Y2Ti2O7 are well correlated. As the Zn content (x) increases, the VB XPS spectra shift to low energies, with an increment in partial states at zero eV, which induces an increase in the carrier charge. UV–visible absorption measurements using the Tauc plot approximation show a decrease in the optical band gap due to an increased concentration of Zn content.