Appearance of Fröhlich-like phonon mode and defect dynamics in La3+-doped ceria

Abstract

The underlying physics which explains the role of cationic and anionic defect dynamics in determining the catalytic activity and ionic conductivity of aliovalent ion-doped ceria is complex and not yet fully understood. To address this issue, we have investigated the structural evolution of LaxCe1-xO2–δ, where x = 0.1, 0.2, 0.3, 0.4, and 0.5. Rietveld refinements of the X-ray diffraction patterns reveal that the crystal structure belongs to a disordered fluorite phase for all samples. We find a systematic increase in the lattice parameter along with a gradual decrease in oxygen site occupancy with the increase in La3+ doping concentration. In Raman spectra of doped compounds, other than well-known crystalline F2g and defect related modes, we observe the presence of an additional Raman mode. The Raman line-shape analysis indicates the non-resonant Fröhlich character of this mode. Temperature dependent Raman measurements demonstrate unique characteristics of this Raman peak. In addition, from the change in relative intensity ratio of the Raman modes related to different types of defect states, we show that vacancies cannot be considered as isolated defects beyond a critical doping level. We propose that the evolution of Raman intensities of defect related modes and Fröhlich mode with the doping level can be used as a marker to determine the role of electron-phonon coupling and anion vacancies in the catalytic activity of doped ceria. Furthermore, by studying the photocatalytic measurements for La3+ doped ceria compounds, we demonstrate that the anion vacancies do not always play a direct role in controlling functional properties.