Raman and x-ray studies of Ce1−<i>x</i>RE<i>x</i>O2−<i>y</i>, where RE=La, Pr, Nd, Eu, Gd, and Tb

TLDR

In the low‑doping regime, trivalent‑RE doped CeO₂ forms solid solutions that preserve the fluorite structure, with lattice constants scaling roughly with the dopant ionic radius. The authors synthesized Ce₁₋ₓREₓO₂₋ᵧ (0 ≤ x ≤ 0.5) from nitrate solutions, characterized the samples by X‑ray diffraction and Raman scattering, and used a Green’s‑function point‑defect model to simulate the effect of oxygen vacancies on the Raman spectrum. Raman spectroscopy revealed that the 465 cm⁻¹ mode shifts to lower frequency with increasing dopant concentration, but after correcting for lattice expansion the shift is positive; the mode also broadens, becomes asymmetric with a low‑frequency tail, and a new broad feature appears near 570 cm⁻¹, all of which are attributed to oxygen vacancies created by trivalent RE substitution.

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.