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 les s-than-or-equal-to 0.5 starting from nitrate solutions of the rare ear ths. X-ray diffraction and Raman scattering are used to analyze the sa mples. These compounds, at least in the low doping regime and for stri ctly trivalent dopants, form solid solutions that maintain the fluorit e structure of CeO2 with a change in lattice constant that is approxim ately proportional to the dopant ionic radius. The single allowed Rama n 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 ex pansion, the frequency shift is actually positive for all the strictly trivalent ions. In addition, the Raman line broadens and becomes asym metric with a low frequency tail, and a new broad feature appears in t he spectrum at approximately 570 cm-1. These changes in the Raman spec trum are attributed to O vacancies, which are introduced into the latt ice whenever a trivalent RE is substituted for Ce4+. This conclusion i s supported by a simple model calculation of the effects of O vacancie s on the Raman spectrum. The model uses a Green's function technique w ith the vacancies treated as point defects with zero mass.