Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms

Optical, structural properties and redox were studied for Ce-bearing silicate and aluminosilicate glasses and Ce-activated silica (SiO₂) fiber preforms, by X-ray Absorption, Raman and Photoluminescence Spectroscopy.

The introduction of Al₂O₃ in Ce-bearing silicate glass strongly stabilizes reduced species in agreement with the optical basicity concept and additionally induces modifications in the Ce^{3 +} surrounding. In Al-free silica preforms, depending on the collapsing conditions, the Ce^{3 +}/ΣCe ratio has the highest variability (Ce^{3 +}/ΣCe = 0.7-0.85 ± 0.07), whereas in Al-bearing fiber preforms, Al codoping induces a strong stabilization of Ce^{3 +} species, despite the collapsing atmosphere.

Al presence induces modifications on Ce local environment, and thus variations of the distance between Ce^{3 +} and its ligands, and/or to variations of the covalency of the bonds. Absorption bands in the UV region, ascribed to Charge-transfer (CT) bands, in our samples, occur only for low amount of Ce^{4 +} (Ce^{3 +}/ΣCe > 0.85 ± 0.07). Indeed, in silicate and aluminosilicate glasses the higher amount of Ce^{4 +} does not give rise to UV absorption bands ascribed to CT processes, whereas, in fiber preforms, Ce^{3 +} species can induce the trapping of hole centers, and enhance the photoluminescence efficiency. More generally, these results enhance our understanding of REE structural and chemical roles in amorphous materials and increase our knowledge for technical applications.