EXAFS studies of the local structure of Er³⁺ ions in silica xerogels co-doped with aluminium

The local environment around Er³⁺ ions in wet and densified (at 900°C) silica xerogels (pure and co-doped with aluminium) has been studied at the Er L₃-edge by X-ray absorption spectroscopy using the fluorescence detection technique. The radial distribution functions (RDF), reconstructed from X-ray absorption fine structure (EXAFS), show several changes in the local co-ordination of erbium ions upon densification: shortening of the Er-O and Er-Si/Al distances, decrease of the co-ordination numbers and broadening of the Er-O RDF. The effect of Al co-doping is clearly discerned by EXAFS in both the first and second co-ordination shells for densified gels and mainly in the second shell for wet gels. For increasing Al content, the interatomic distances between erbium ions and the second co-ordination shell ions become longer and have narrower distribution. A preferential bonding of Er to Al ions is clearly detected, with increasing ordering at higher Al content. EXAFS does not show evidence of a clustering for Er³⁺ ions after densification: the short range Er-Er co-ordination is absent or not detectable in the present experiments.