Cerium clustering and radiation damage resistance in aluminophosphate and silicophosphate glasses

Cerium oxide is a well-known additive for increasing resistance to radiation damage in glass by preventing electrons and holes freed by irradiation from becoming trapped at defect sites and inducing optical absorption bands which can severely darken the glass. Phosphate glasses provide a unique opportunity for studying radiation damage resistance due to their high rare-earth solubility, ~25 mol%. Two series of glasses, nominally AlP3O9-CeP3O9 and CeP3O9-SiP2O7, were synthesized to investigate structure-property relationships in a range of compositions near the metaphosphate. The presence of cerium clustering, or sharing of oxygen between cerium cations, was predicted using the chain fragment cluster model, an extension of earlier models for rare-earth phosphate glasses. Using the atom% composition determined by XPS from vacuum fracture surfaces, and cation coordination measured by Ce K-edge EXAFS, 29Si CPMG NMR, and 27Al MAS NMR, it was determined that clustering occurs for glasses containing ≥ 14 mol% Ce2O3 in the aluminophosphate glass series and ≥ 18 mol% Ce2O3 in the silicophosphate glass series. Many measured properties have been observed to correlate with the presence or absence of cerium clustering, cluster size, or other concomitant structural changes, including: visible coloration, density, refractive index, Ce3+ photoluminescence, and Ce3+ paramagnetic resonance. Additionally, radiation damage resistance was identified in the aluminophosphate and silicophosphate glasses which were predicted to have clustered cerium cations through the absence of radiation-induced phosphorus-related paramagnetic defects. This resistance is attributed to a structural implication of clustering. Specifically, cerium cations will be in close proximity to defect precursor sites at the concentrations required for clustering and are thus able to prevent localization of electrons and holes on those sites. Finally, irradiation-induced optical absorption was measured in all glass compositions and was attributed to a change in the local electronic structure of cerium which is similar to intervalence charge transfer. This induced absorption provides evidence for the mechanism of radiation damage resistance where cerium acts as a preferential electron- or hole-trap and either changes oxidation state or forms Ce3++ and Ce4++e-.