Identifying Defects in Ceria-Based Nanocrystals by UV Resonance Raman Spectroscopy

Local structures in ceria-based materials were investigated by UV resonance Raman spectroscopy using a 363.8 nm laser line for the first time. In the spectra of a highly crystalline and undoped ceria, overtone LO series are clearly detected due to the multiphonon relaxation by the UV resonance Raman effect. Gd3+ doping in ceria additionally activates the disorder bands that are attributable to lattice spaces with or without an oxygen vacancy. The crystal size dependency of the UV Raman spectra suggests that Ce3+ ions preferably form the MO8-type (M = Ce3+) complex in the undoped ceria nanoparticles and are eliminated by oxidation while heating in air. Gd3+ ions also tend to form a MO8-type complex in the nanocrystalline ceria matrix. However, they diffuse to form the more thermodynamically stable defect clusters that accompany the grain growth upon heating.