Evidence of charge delocalization in Ce1−xFex2+(3+)O2−ynanocrystals(x=0, 0.06, 0.12)

Abstract

We have measured Raman scattering spectra of pure and iron-doped Ce${}_{1\ensuremath{-}x}$Fe${}_{x}$${}^{2+(3+)}$O${}_{2\ensuremath{-}y}$ ($x=0$, 0.06, and 0.12) nanocrystals. According to the x-ray diffraction study, Fe doping produces contraction of the CeO${}_{2}$ 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 CeO${}_{2\ensuremath{-}y}$ samples are not only localized at Ce${}^{3+}$ ions but also delocalized onto Ce(Fe)-O(V${}_{O}$)-Ce(Fe) orbitals. Delocalization of electrons from Ce${}^{3+}$ 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 CeO${}_{2\ensuremath{-}y}$. The electron-molecular vibration (phonon) coupling constants $\ensuremath{\lambda}$ 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.