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An experimental investigation of the crystallographic, Raman and transport properties of the Ce1−x(Nd0.74Tm0.26)xO2−x/2 (0.1 ≤ x ≤ 0.6) doped ceria system was performed with the aim of setting out correlations between structural features and ionic conductivity of the material. The chosen composition ensures that the average size of the Nd3+ and Tm3+ doping ions coincides with the one of Sm3+; even so, the studied system presents larger cell parameters and a wider compositional extent of the CeO2-based solid solution than Sm-doped ceria. Moreover, the occurrence of two different activation energies to ionic conduction below and above ~750 K determines the existence of two distinct conduction regimes. The described experimental results agree with the formation below the threshold temperature of 1 V O . . 2 T m C e ' trimers, which promote the incorporation of Nd C e ' isolated defects into the CeO2-based solid solution. In the high temperature range the dissociation of trimers induces the appearance of a lower activation energy; the extrapolation of its value at infinite dilution provides a result in good accordance with the expected binding energy of 1 V O . . RE C e ' dimers, pointing at their stability even in the high temperature conduction regime.

Defect aggregates in doped ceria play a crucial role in blocking the movement of oxygen vacancies and hence in reducing ionic conductivity. Nevertheless, evaluation of their amount and the correlation between domain size and transport properties is still an open issue. Data derived from a high-pressure X-ray diffraction investigation performed on the Ce1–x(Nd0.74Tm0.26)xO2–x/2 system are employed to develop a novel approach aimed at evaluating the defect aggregate content; the results are critically discussed in comparison to the ones previously obtained from Sm- and Lu-doped ceria. Defect clusters are present even at the lowest considered x value, and their content increases with increasing x and decreasing rare earth ion (RE3+) size; their amount, distribution, and spatial correlation can be interpreted as a complex interplay between the defects’ binding energy, nucleation rate, and growth rate. The synoptic analysis of data derived from all of the considered systems also suggests that the detection limit of the defects by X-ray diffraction is correlated to the defect size rather than to their amount, and that the vacancies’ flow through the lattice is hindered by defects irrespective of their size and association degree., A novel approach to the treatment of high-pressure X-ray diffraction data is applied to several rare earth-doped ceria systems with the aim of providing an evaluation of the amount of defect aggregates and of the composition of the CeO2-based solid solution. By this method, it is possible to effectively correlate the structural properties and ionic conductivity of the studied material.

As a result of the lattice mismatch between the oxide itself and the substrate, the high-pressure structural properties of trivalent rare earth (RE)-doped ceria systems help to mimic the compressive/tensile strain in oxide thin films. The high-pressure structural features of Sm-doped ceria were studied by X-ray diffraction experiments performed on Ce1&minus, xSmxO2&minus, x/2 (x = 0.2, 0.3, 0.4, 0.5, 0.6) up to 7 GPa, and the cell volumes were fitted by the third order Vinet equation of state (EoS) at the different pressures obtained from Rietveld refinements. A linear decrease of the ln B 0 vs. ln ( 2 V a t ) trend occurred as expected, but the regression line was much steeper than predicted for oxides, most probably due to the effect of oxygen vacancies arising from charge compensation, which limits the increase of the mean atomic volume ( V a t ) vs. the Sm content. The presence of RE2O3-based cubic microdomains within the sample stiffens the whole structure, making it less compressible with increases in applied pressure. Results are discussed in comparison with ones previously obtained from Lu-doped ceria.

Transport and structural properties of heavily doped ceria can reveal subtle details of the interplay between conductivity and defects aggregation in this material, widely studied as solid electrolyte in solid oxide fuel cells. The ionic conductivity of heavily Gd-doped ceria samples (Ce1−xGdxO2−x/2 with x ranging between 0.31 and 0.49) was investigated by impedance spectroscopy in the 600−1000 K temperature range. A slope change was found in the Arrhenius plot at ~723 K for samples with x = 0.31 and 0.34, namely close to the compositional boundary of the CeO2-based solid solution. The described discontinuity, giving rise to two different activation energies, points at the existence of a threshold temperature, below which oxygen vacancies are blocked, and above which they become free to move through the lattice. This conclusion is well supported by Raman spectroscopy, due to the discontinuity revealed in the Raman shift trend versus temperature of the signal related to defects aggregates which hinder the vacancies movement. This evidence, observable in samples with x = 0.31 and 0.34 above ~750 K, accounts for a weakening of Gd−O bonds within blocking microdomains, which is compatible with the existence of a lower activation energy above the threshold temperature.