Defect evolution in Eu3+, Nb5+ doped and co-doped CeO2: X-ray diffraction, positron annihilation lifetime and photoluminescence studies

Doping of ceria with trivalent lanthanides and pentavalent ions is known to influence its catalytic activity, optical properties and ionic conductivity. In this study, (Nb,Eu)-codoped CeO2 has been synthesized by hydroxide precipitation to study the synergistic effect of (Nb, Eu) codoping on the defect evolution and optical properties of CeO2. Through X-ray diffraction, it was seen that Nb5+ has limited solubility in CeO2 which could be enhanced by codoping with Eu3+. From Rietveld refinement of the X-ray diffraction data, the ceria lattice seems to expand on Eu3+ doping which could be explained taking into account the formation of oxygen vacancies (OVs) in stoichiometric ratios. The observed lower than expected contraction in the Nb5+ doped samples is attributed to lattice expansion due to the creation of oxygen interstitials (OIs). PL studies in terms of the ratio of electric dipole transition (EDT) to magnetic dipole transition (MDT) intensities have shown that Eu3+ lacks inversion symmetry at Ce4+ which invokes the formation of OVs for charge compensation. Interestingly, Nb5+ co-doping restored the inversion symmetry around the Eu3+ site by eliminating the formed OVs. This is achieved by ion pair matching of the dopants. This work has great relevance in understanding the role of defects and co-doping in designing highly efficient catalysts, solid oxide fuel cells and doped phosphors.