Local order around Er3+ ions in SiO2–TiO2–Al2O3 glassy films studied by EXAFS

The structural environments of Er3+ ions were studied by extended X-ray absorption fine structure (EXAFS) measurements above the Er LIII edge, in SiO2–TiO2–Al2O3 sol–gel derived glassy films with varying Er and co-dopant (Al) contents. The 4I13/2 excited state lifetimes of the Er3+ ion were also measured for selected samples containing equal amounts of Er2O3 and Yb2O3, the latter being added as a fluorescence sensitizer. Both the first and the second Er3+ co-ordination shells were analyzed. The first co-ordination shell was composed of ∼6–7 oxygen atoms at distances varying between 0.225 and 0.229 nm, with small Debye–Waller factors, varying between 1.2 and 2.2×10−4 nm2. The second shell was basically composed of tetrahedrally co-ordinated second near neighbor (SNN) atoms, bonded through corner-sharing Er–O–SNN bridges. The SNN species were found to be either Si, or a combination of Si and Al, depending on the matrix composition. Although the measured lifetime values for the Er3+ fluorescence at 1.54 μm varied between ca. 0.5 and 5.0 ms, depending on the Al content and indicated the occurrence of Er3+ concentration quenching above 0.5 at.% Er in films co-doped with 9% Al, no Er–Er interactions were detected by EXAFS in films doped with up to 1.75% Er, within the sensitivity of this technique (∼0.5 nm). This suggests that there was no rare-earth (RE) clustering in the present films, but that certain mechanisms of RE ion–ion interactions at longer distances were active.