Structural characterization of Tb- and Pr-doped ceria
Introduction
Doped ceria materials (Ce1 − xRExO2 − y, RE = Y and Rare Earths) find application as electrolytes in Solid Oxide Fuel Cells (SOFCs) technology. The best performances are generally observed for those dopants (Gd, Sm, Y) that minimize the size mismatch with the host Ce+ 4, thus reducing defects aggregation and facilitating the diffusion process [1]. The increase of the doping amount above a certain critical threshold, around x = 0.20 [1], otherwise produces dopant rich nanodomains [2], [3], with the tendency of the dopant, probed by EXAFS [4], [5], [6], NMR [7] and PDF [3], [8], to retain the local environment as in its pure oxide. Another common feature of all doped-ceria systems is the presence of ~ 8-fold coordinated Ce in the + 4 state and ~ 7-fold coordinated dopant ions in the + 3 state [4], [9]. On the opposite, RE = Pr and Tb dopants do not obey this rule. Literature reports on Pr and Tb oxidation state in ceria are often controversial, thus leading to the general interpretation that they exhibit a mixed valence state. In details, a predominant + 3 state was found for Pr in [10], [11], [15] and Tb in [12], whereas others found Pr [6], [9], [13], [16] and Tb [14] mostly in their + 4 state. Such variable valence behavior rebounds on special optical properties, making Pr- and Tb-doped ceria suitable for application as environmental friendly ceramic pigments [17]. The aim of this work is then to perform a systematic investigation of Tb- and Pr-doped ceria systems, discussing their crystallographic structure and optical properties in the framework of those of other trivalent cations RE(= Yb, Y, Gd, Sm, Nd, La). In this context, X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD) were combined to assess a complementary structural study, exploiting the PDF of XRPD data for investigating the local structure. In addition, energy dispersive X-ray (EDX) microprobe was employed to determine the metal stoichiometry and UV–visible spectroscopy for collecting absorbance spectra.
The large variability in the findings on Tb- and Pr-based compounds is often ascribed to the different methods employed for sample preparation. For example, aging temperature has a strong impact on the cation diffusivities in the fluorite structure, thus likely affecting the distribution of the cations, especially when a solid state reaction synthesis is applied. In this work, in order to perform a consistent comparison between Tb- and Pr-doping, we consider samples obtained following exactly the same, wet, synthesis route. The latter was chosen for facilitating sample homogenization.
Since large Tb-doping into ceria is reported to produce biphasic samples (see e.g. [18]), Tb- and Pr-doped ceria systems are compared up to the x = 0.500 doping concentration. On the contrary, the whole composition range of the ceria-praseodymium oxide system is discussed.
Section snippets
Material and methods
Ce1 − xRExO2 − y (RE = Pr, Tb) samples with different x RE-concentration were prepared with the Pechini sol–gel method [19] and fired at 900 °C for 72 h. CeO2 (> 99 %) and Tb2O3 (99.99%) were purchased by Aldrich. The metal elemental composition was determined using an EDX microprobe mounted on a LEO-1430 SEM instrument. The actual and nominal concentrations of all the samples investigated are given in Table 1. XRPD patterns for Rietveld analysis were collected at room temperature (RT) at the ID31
Effect of different dopants
A first glance at doped samples reveals that Pr- and Tb-doped ceria feature a brownish color, whereas pure CeO2 and the other doped samples are pale yellow, with the only exception of Nd, which shows a greenish shade. The UV–vis absorption spectra of pure CeO2 and of all RE250 doped samples are shown in Fig. 1 (a). All the samples, but the ones containing Pr and Tb, feature a similar absorption band limited to λ < ~ 400 nm. This can be ascribed to O− 2 to Ce+ 4 charge transfer and interband
Conclusions
A set of Pr- and Tb-doped ceria samples has been systematically investigated and compared with other dopants.
The optical properties of the samples investigated are in line with the literature reports. The special optical behavior induced by Tb- and Pr-doping in ceria, compared to other dopants, is explained in terms of their different oxidation states. For doping rates up to x = 0.500, Tb- and Pr-doped ceria retain the fluorite structure of cerium oxide, though behaving differently: Tb ions show
Acknowledgments
The authors acknowledge financial support by the Fondazione CARIPLO: Project n. 2010-0612. They also acknowledge the European Synchrotron Radiation Facility and the Institut Laue-Langevin for provision of beam time. They also wish to thank Ms. B. Sacchi for the EDX data collection; Dr. M. Dapiaggi for the collection of laboratory XRPD patterns and Dr. G. Vaughan for useful discussions.
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