Defect aggregates and transport properties in the Ce1-x(Nd0.74Tm0.26)xO2-x/2 system

Unraveling the correlations among crystal structure, presence of defects, and properties in functional materials, is one of the most challenging and fascinating tasks for a materials scientist. Within this framework, rare earth (RE)-doped ceria Ce1-xRExO2-x/2 comes as a treasure trove of connections between subtle structural details and transport properties, that can be unveiled also thanks to synchrotron x-ray diffraction. 
Due to its high values of ionic conductivity in the intermediate temperature range (0.01-0.1 S cm-1 at 873 K), RE-doped ceria is regarded as an excellent solid electrolyte for solid oxide cells (SOCs); among the different systems, Sm-, Gd- and Nd-doped ceria are the most relevant ones in terms of ionic transport. In the present study, the (Nd0.74Tm0.26) mixture was chosen since it presents the same average ionic size as Sm3+, resulting from the contribution of a larger (Nd3+) and a smaller (Tm3+) ion.
SOCs are electrochemical devices able to work as fuel cells, namely to convert the chemical energy of a fuel into electricity, but also to operate in the reverse direction as electrolysis cells, thus converting a (CO2+ H2O) mixture into syngas (CO + H2) by supplying energy to the cell. The electrolyte is expected to favor the migration of oxygen ions through the lattice toward the anode. In RE-doped ceria ionic conduction occurs by hopping of O2- ions over vacancies generated by the partial replacement of Ce4+ with RE3+ ions, and conductivity is ruled by an Arrhenius-like behaviour, where activation energy to ionic conduction (Ea) can be deemed as the sum of migration enthalpy and association enthalpy. While the former is the energy needed by free vacancies to move through the lattice, the latter is the energy difference between isolated defects and defect clusters, i.e.the energy necessary to release vacancies from defect clusters. But what is meant by defects and defects clusters, and which is their role in the ionic conduction of doped ceria? To answer this question, the atomic arrangement must be taken into account. CeOcrystallizes in a fluorite-like cubic structure belonging to the space group Fm-3m, generally called F. Within this cell Ce is eight-coordinated to O. At least at low substitution degree, the Ce4+ partial replacement gives rise primarily to a solid solution hosting isolated RE’Ce defects and oxygen vacancies, with the latter being free to move at a sufficiently high temperature. Nevertheless, beyond a certain threshold, depending on RE, diffraction patterns present superstructure peaks referable to the Ia-3space group, namely the signature of the cubic phase (called C) typical of the RE3+-vacancy clusters, where RE is six-coordinated to O. The appearance of these defect aggregates is responsible for the drop in ionic conductivity observed at higher than 0.15-0.20: within this structure vacancies are blocked at fixed crystallographic positions. The fundamental units building the C-based defect aggregates are 1V0●●RE’Ce positively charged dimers and 1V0●●2RE’Ce neutral trimers, with the binding energy of the latter increasing with decreasing the RE3+ size.
Two main results derive from the analysis of the structural and transport properties of the (Nd,Tm)-doped system, performed at the MCX beamline of Elettraand in the framework of the project COELUS (Production of renewable fuel by CO-ELectrolysis and reUSe of carbon dioxide) funded by Compagnia di San Paolo and recently published in J. Energy Chem. The first and foremost evidence inferable from diffraction patterns is the larger cell parameter with respect to the Sm-doped system. The second evidence, obtained by impedance spectroscopy, is the observed discontinuity in the Arrhenius plot at ⁓750 K (see Figure 1), which allows to recognize the existence of two different activation energies, with the higher one at lower temperature. Both results suggest the same conclusion, namely that the prevailing defect clusters are 1V0●●2RE’Ce trimers. With respect to dimers, trimers are in fact characterized by a stronger dependence of their binding energy on the RE3+size, making 1V0●●2Tm’Ce associates much more stable than 1V0●●2Nd’Ce; therefore, RE3+remaining within F essentially consists of Nd3+, which is larger than Sm3+, thus causing the occurrence of the larger cell size. Moreover, the predominant formation of trimers is also in agreement with the existence of a lower Eat higher temperature, due to the expected dissociation of these clusters above a certain temperature, caused by their lower configurational entropy.


Figure 1 Arrhenius plots of ionic conductivity (s) of the Ce1-x(Nd0.74Tm0.26)xO2-x/2system. Dashed lines are regression lines fitting experimental data.




This research was conducted by the following research team:

Cristina Artini1,2, Sabrina Presto3, Massimo Viviani3, Sara Massardo1, Maria Maddalena Carnasciali1,4, Lara Gigli5and Marcella Pani1,6

Department of Chemistry and Industrial Chemistry, University of Genova, Genova, Italy
Institute of Condensed Matter Chemistry and Technologies for Energy, CNR-ICMATE, Genova, Italy
Institute of Condensed Matter Chemistry and Technologies for Energy, CNR-ICMATE, c/o DICCA-UNIGE, Genova, Italy
INSTM, Genova Research Unit, Genova, Italy
Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
CNR-SPIN Genova, Genova, Italy

Contact persons:

Cristina Artini, email:


C. Artini, S. Presto, M. Viviani, S. Massardo, M.M. Carnasciali, L. Gigli, and M. Pani, “The role of defects association in structural and transport properties of the Ce1-x(Nd0.74Tm0.26)xO2-x/2system”, J. Energy Chem. 60, 494 (2021); DOI: 10.1016/j.jechem.2020.11.030.

Last Updated on Monday, 28 June 2021 16:25