Raman and x-ray studies of Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb

Abstract

Powdered samples of the type Ce1−xRExO2−y, where RE=La, Pr, Nd, Eu, Gd, and Tb, are synthesized over the range 0≤x≤0.5 starting from nitrate solutions of the rare earths. X-ray diffraction and Raman scattering are used to analyze the samples. These compounds, at least in the low doping regime and for strictly trivalent dopants, form solid solutions that maintain the fluorite structure of CeO2 with a change in lattice constant that is approximately proportional to the dopant ionic radius. The single allowed Raman mode, which occurs at 465 cm−1 in pure CeO2, is observed to shift to lower frequency with increasing doping level for all the rare earths. However, after correcting for the Grüneisen shift from the lattice expansion, the frequency shift is actually positive for all the strictly trivalent ions. In addition, the Raman line broadens and becomes asymmetric with a low frequency tail, and a new broad feature appears in the spectrum at ∼570 cm−1. These changes in the Raman spectrum are attributed to O vacancies, which are introduced into the lattice whenever a trivalent RE is substituted for Ce4+. This conclusion is supported by a simple model calculation of the effects of O vacancies on the Raman spectrum. The model uses a Green’s function technique with the vacancies treated as point defects with zero mass.

Potential rare-earth modified CeO2 catalysts for soot oxidation

“…In the case of La 3+ and Sm 3+ modified catalysts, 2h shifted to lower values, suggesting the incorporation of the relatively larger ions into CeO 2 cubic structure forming the solid solution [13]. Similar observations are made for catalysts prepared by 700°C calcinations.…”

“…The surface area of CeO 2 is found to be 57 to 2 m 2 g )1 , when calcined at 700 and 1000°C, respectively. The rare-earth ions selected in this study are known to form solid solution and, as a consequence, also stabilise the CeO 2 crystallites towards sintering, resulting in a less severe surface area decline [13]. However, the surface area decreased for CeREO x _700 catalysts, compared with CeO 2 _700.…”

Role of Hydroxyl Groups in the Preferential Oxidation of CO over Copper Oxide–Cerium Oxide Catalysts

“…25,26 The position of this band provides information about the introduction of foreign cations into the ceria lattice. The position of the F 2g band in pure ceria is usually ca 464 cm -1 , 20 and the position obtained for all CuO/Ce 0.8 X 0.2 O δ catalysts is well below this value (see data in Table 1).…”

Microstructural studies of bulk and thin film GDC

“…9a. The Raman-active mode in this material corresponds to a frequency of ω R ∼466 cm −1 , which is attributed to a symmetrical stretching mode of the Ce-O8 vibrational unit of cubic ceria [16,17]. Therefore, this mode should be very sensitive to any disorder in the oxygen sublattice resulting from thermal and/or grain size induced nonstoichiometry.…”