Investigation of in-situ oxygen vacancies dissociation mechanism and associated atomic scale reshuffling during oxy-ion migration in nanostructured co-doped ceria

The atomic scale reshuffling during oxygen ion conductivity in co-doped ceria lattice at intermediate temperature (400-700 degrees C) range are investigated by in-situ measurement using spectroscopic tools EXAFS, XANES, Raman and XRD. Oxy-ion hopping site decide migration barrier through ceria lattice and thus activation energy for conductivity; present interest is to extrapolate ionic conductivity by exploring oxy-ion hopping site. Hydrothermally synthesized nanocrystalline Sm-based aliovalent and isovalent co-doped ceria systems with optimized dopant content are used for the study. Charge-size effect of aliovalent and isovalent co-dopant pair on ceria lattice are revealed from Rietveld refinement of XRD data. HR-TEM images confirmed nanoscale dimensions (similar to 15 nm) of grains of as-calcined as well as sintered samples (100-150 nm) with well defined grain boundaries. HT-EXAFS data are extracted to get information of co-ordination number (CN), interatomic spacing (IS) and atomic disorder at 1st shell (NN) and 2nd shell (NNN) site. The rate of change of CN and IS with temperature are closely associated with the rate of oxygen vacancies (i) formation, (ii) relaxation and (iii) dissociation at NN and NNN site of cations; which lead to atomic scale reshuffling at NN and NNN site of cations. Our work demonstrates, in aliovalent doped ceria, NN-site favour oxygen vacancies dissociation; while for isovalent dopant pair, NNN-site is proactive for oxygen vacancies dissociation. HT-Raman spectra also supported these results by disappearance of respective defect associated Raman mode. HT-XANES spectra revealed intrinsic oxygen vacancies dominancy in aliovalent co-dopant. The oxygen ion migration route through NN-site is found lower energetic than that of NNN-site as-confirmed from ionic conductivity data. The ionic conductivities of Sm-Ca and Sm-Sr aliovalent codoped ceria systems are obtained one order more than Sm-Gd, Sm-Dy and Sm-Nd isovalent co-doped ceria systems. As well as activation energy of aliovalent co-doped ceria is lower than isovalent once.