Temperature Dependent near Infrared Spectroscopy of Electron Irradiated Ceria Single Crystals

The FTIR absorption bands of virgin and electron-irradiated CeO₂ single crystals were measured from 20 K to 500 K between 4000 cm^-1 and 12,000 cm^-1 (~830 nm to 2500 nm). Three broad bands centered at about 6100 cm^-1 (~0.75 eV), 7000 cm^-1 (~0.87 eV), and 10,500 cm^-1 (~1.3 eV) were recorded above 100 K for the 2.5 MeV electron energy. Two smaller bands at about 4300 cm^-1 (~0.53 eV) and 5500 cm^-1 (~0.68 eV) were also recorded below 100 K. Similar broad bands centered at about 4100 cm^-1 (~0.52 eV), 6400 cm^-1 (~0.79 eV), 7600 cm^-1 (~0.94 eV), and 10,500 cm^-1 (~1.3 eV) are also found for the 1.4 MeV electron energy above 300 K. The evolution of these absorption bands was followed as a function of temperature. The plots of band intensity ratios show a thermally activated process corresponding to the ionization of the deep electronic levels of point defects in the band gap of ceria of ~26,000 cm^-1 (~3.2 eV). These five bands are assigned to the different charge states (0, -1, -2, -3, -4) of the Ce vacancies produced by elastic collisions above 1.0 MeV.