Recovery of damage in electron-irradiated ceria

Abstract

The damage recovery of an electron-irradiated CeO2 single crystal was studied by in situ UV–visible absorption spectroscopy during isochronal thermal annealing up to 873 K. The spectrum of the as-irradiated crystal shows band tailing below the fundamental absorption edge for photon energy near 3.2 eV. Two broad Gaussian bands centered at about 0.9 and 1.4 eV are also recorded. The edge region is treated by including the Urbach edge contribution. The variation of bandgap energy and Urbach energy is followed as a function of the annealing temperature. The absorption edge was red-shifted with annealing temperature then returned to the starting value after cooling back to room temperature. The disorder contribution to the Urbach edge is deduced by subtracting the temperature contribution. It shows a decay of the disorder contribution following a first-order kinetics process with an onset of recovery at 373 K and activation energy of about 0.15 eV. Full recovery is achieved at 873 K, whereas a partial recovery of the two absorption bands (at 0.9 and 1.4 eV) is found. The low activation energy is interpreted in reference to the available literature data on the energy barriers for point defect migration, and assigned to the migration of oxygen interstitials associated to electronic levels near the top of the valence band edge. A recombination mechanism is suggested, involving both electron hopping and interstitial oxygen diffusion.