Resonant Raman Scattering in Undoped and Lanthanide-Doped CeO2

Abstract

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 Frohlich 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 c...