Irradiation response during the early stages of alpha radiation damage in mesoporous nanocrystalline ceria films
Résumé
We investigated the effect of alpha radiation on mesoporous ceria in the very early stages of damage caused by electronic energy loss processes. Most studies of radiation effects in nanocrystalline ceria have been conducted on relatively dense samples and in the ballistic regime of double collisions. Few studies on ceramic systems have focused on radiation effects in loosely packed nanocrystalline systems with predominantly free surfaces. Free surfaces are considered crucial for radiation resistance, yet few studies have specifically investigated radiation-induced changes in structural correlations in such systems. We irradiated mesoporous nanocrystalline CeO2 films with well-characterized architectures with 800 keV He+ ions. Glancing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and diffuse reflectance spectroscopy were used to measure quantitative changes in structural correlations and modifications in electronic properties as a function of ion fluence. Small polarons are produced by He+ ion irradiation; they are stabilized by strong electron-phonon coupling and are not fully annealed at the surface of the nanocrystals; they alter the average charge of sites in the crystal by deforming their local environment. The concentration of these polaron defects increases with He ion fluence but nevertheless remains low: their concentration at the highest fluence hardly leads to an increase in the lattice parameters and does not manifest itself as microstrain. Irradiation induces changes in the formal valence, as illustrated by the optical spectroscopy results. This can be understood in the extended Hubbard scheme: key components of this model are the Hubbard parameters: cerium oxide suffers from a large on-site repulsion energy (approximately 5 eV) due to self-interaction. The Hubbard functional aims to correct the energy curvature with respect to the population of the Hubbard manifold. In the polaron picture, a charge fraction localizes on the cerium atoms and thus moves away from the oxygen atoms. This generates a corresponding relative shift correction of the oxygen 2p states and the cerium f states, resulting in a change in the optical response that depends on the polaron concentration. In this respect, these mesoporous cerium oxide systems exhibit excellent resistance to alpha particle radiation. On the other hand, polarons have a measurable influence on the phonon lifetime and band structure, thus affecting technological properties of these systems, such as thermal conductivity. We can assume that the effects and mechanisms observed in cerium oxide nanocrystals during the early stages of alpha particle irradiation are transferable to other mixed-valence oxides whose conductivity is polaronic in nature, and specifically to uranium, plutonium, and americium oxides.