Resonant Raman Scattering in Undoped and Lanthanide-Doped CeO₂

CeO₂ has a narrow, empty band of Ce 4f states that lies between an O 2p-based valence band and a Ce 5d-based conduction band. The O 2p-Ce 4f optical band gap is positioned at ∼3.2 eV with an absorption band centered at ∼3.8 eV. We investigated the Raman scattering of bulk CeO₂ in the excitation energy range of 1.96-3.81 eV. The resonant enhancement profile of the longitudinal optical (LO) phonon at ∼590 cm^-1 closely follows that of the 2LO band and both profiles track the optical absorption of the O 2p-Ce 4f electronic transition. Multi-LO phonon bands were found to appear up to the sixth order, pointing to an electron-phonon Fröhlich interaction as the source of the resonant enhancement. The ∼600 cm^-1 off-resonant D₂ band (denoted as MO₈-type complex in ceria doped with M aliovalent ions) is overshadowed under resonant conditions by the resonant LO phonon scattering. Hence, spectral analysis of defect bands under resonant conditions has to be distinct from that applied under off-resonant conditions and care must be taken when dealt with under a single framework. We investigated the resonant Raman spectra of Lu-, La-, Gd-, or Sm-doped ceria ceramic pellets as a function of increasing Do³⁺ mol %, in the fluorite phase range (up to 20 mol %). For La and Lu, the general trend of the Do³⁺ mol % frequency dependence for the D₁ local mode is qualitatively similar to that of the F_2g phonon and it follows the respective expansion (La) or contraction (Lu) in the lattice parameter. However, for Gd and Sm, the trend is opposite to the F_2g mode. This trend may stem from local lattice contraction around point defects, which was suggested, based on local structure probes such as X-ray absorption spectroscopy and pair distribution function analysis of X-ray diffraction. Our analysis provides access to average as well as to local structures of ceria solid solutions, via resonant Raman spectroscopy.