Cerium metal oxidation studied by IR reflection-absorption and Raman scattering spectroscopies

Oxidation of cerium metal is a complex process which is strongly affected by the presence of water vapor in the oxidative atmosphere. Here, we explore, by means of IR reflection-absorption (IRRAS) and Raman scattering spectroscopies, thin oxide films, formed on cerium metal during oxidation, under dry vs. ambient (humid) air conditions (~0.2% and ~50% relative humidities, respectively) and compare them with a thin film of CeO2 deposited on a Si substrate. Complementary analysis of the thin films using X-ray diffraction and Focused Ion Beam-Scanning Electron Microscopy enables the correlation between their structure and spectroscopic characterizations. The initial oxidation of cerium metal results in the formation of highly sub-stoichiometric CeO2-x. Under dry air conditions, a major fraction of that oxide reacts with oxygen to form CeO~2, which is spectroscopically detected by Raman scattering F2g symmetry mode and by IRAAS F1u symmetry mode, splitted into doubly-degenerate Transverse Optic (TO) and mono-degenerate Longitudinally Optic (LO) modes. In contrast, under ambient (humid) conditions, the oxide formed is more heterogenous, as the reaction of CeO2-x diverges towards the dominant formation of Ce(OH)3. Prior to the spectral emergence of Ce(OH)3, hydrogen ions incorporate into the highly sub-stoichiometric oxide, as manifested by Ce-H local vibrational mode detected in the Raman spectrum. The spectroscopic response of the thin oxide layer thus formed is more complex; particularly noted is the absence of the LO mode. It is attributed to the high density of microstructural and compositional defects in the oxide layer, which results in a heterogenous dielectric nature of the thin film, far from being representable by a single phase of CeO~2.