Evidence of charge delocalization in Ce1-xFex2+(3+)O2-y nanocrystals (x=0, 0.06, 0.12)

We have measured Raman scattering spectra of pure and iron-doped Ce1-xFex2+(3+)O2-y (x = 0, 0.06, and 0.12) nanocrystals. According to the x-ray diffraction study, Fe doping produces contraction of the CeO2 unit cell, leading to the Raman mode hardening. Contrary to expectation, the F-2g Raman mode exhibits softening and broadening by changing the valence state of Fe dopant, as a consequence of the electron-molecular vibration coupling. This finding supports the assumption that additional charge in highly oxygen-deficient pure and Fe-doped CeO2-y samples are not only localized at Ce3+ ions but also delocalized onto Ce(Fe)-O(V-O)-Ce(Fe) orbitals. Delocalization of electrons from Ce3+ ions causes insulator-to-metal transition in highly oxygen-deficient nanoceria. The far-infrared reflectivity spectrum of nanoceria shows metalliclike reflectivity, which in the low-frequency region is well fitted with the Hagen-Rubens approximation for metals. Photoluminescence measurements revealed the existence of a defect-related band in the energy gap of CeO2-y. The electron-molecular vibration (phonon) coupling constants lambda and density of electron states at the Fermi level per spin and molecule N(0) were determined within the framework of Allen's theory. The proposal of an energy band structure of nanoceria is also presented.