The following publication attributes a 270 cm-1 of CeO2 nanometer-thick films to a surface phonon mode that is sensitive to the nanometer-scale dimension (i.e., the mode is not present in bulk ceria).  The 514.5 nm line of an argon-ion laser was used as the excitation.  Perhaps this is the same or related?

http://www.sciencedirect.com/science/article/pii/S0254058400003576

Dear Kaoru Aou,

Thank for your answer. The publication you sent is interesting. However, I did not observe the peak at 315 cm-1 (also assigned to surface phonon mode), which is the most prominent peak compared with the other at 270 cm-1. I should add that my sample is not a film but a polycristalline nanopowder with high surface area (220 m2/g) and crystal size of 7 nm. 

Bruno:

I don't work with ceria, but I know a little bit about Raman. If your Raman spectrometer allows you to measure the depolarization ratio of your peak at 250 cm-1, that could offer some more insight into the nature of the mode. For example if the depolarization ratio is small or near zero the mode his highly symmetric. If the depolarization ratio is about 3/4, the mode is asymmetric. Knowledge of the symmetry of the mode can help with narrowing down the possibilities. And of course, working with pure ceria rather than a mixed oxide would be preferred in this context.

I hope this helps!