RAMAN AND X-RAY STUDIES OF CE1-XREXO2-Y, WHERE RE=LA, PR, ND, EU, GD, AND TB

Powdered samples of the type Ce1-xRExO2-y, where RE=La, Pr, Nd, Eu, Gd, and Th, are synthesized over the range 0 less-than-or-equal-to x less-than-or-equal-to 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 Gruneisen 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 approximately 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.