EXAFS studies of the local structure of Er 3 ions in silica xerogels co-doped with aluminium F.Rocca a, * ,M.Ferrari a ,A.Kuzmin b ,N.Daldosso c ,C.Duverger c ,F.Monti d a CeFSA ± Centro CNR-ITC di Fisica degli Stati Aggregati, Via Sommarive 18, 38050 Povo Trento), Italy b Institute of Solid State Physics, University of Latvia, Kengaraga street 8, LV-1063 Riga, Latvia c INFM and Dipartimento di Fisica, Universit  a di Trento, Via Sommarive 14, 38050 Povo Trento), Italy d Dipartimento Scienti®co e Tecnologico, Universit  a di Verona, Strada Le Grazie, 37134 Verona, Italy Abstract The local environment around Er 3 ions in wet and densi®ed at 900°C) silica xerogels pure and co-doped with aluminium) has been studied at the Er L 3 -edge by X-ray absorption spectroscopy using the ¯uorescence detection technique.TheradialdistributionfunctionsRDF),reconstructedfromX-rayabsorption®nestructureEXAFS),show several changes in the local co-ordination of erbium ions upon densi®cation: shortening of the Er±O and Er±Si/Al distances, decrease of the co-ordination numbers and broadening of the Er±O RDF. The eect of Al co-doping is clearlydiscernedbyEXAFSinboththe®rstandsecondco-ordinationshellsfordensi®edgelsandmainlyinthesecond shell for wet gels. For increasing Al content, the interatomic distances between erbium ions and the second co-ordi- nation 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 Er 3 ions afterdensi®cation:theshortrangeEr±Erco-ordinationisabsentornotdetectableinthepresentexperiments. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Thereisgreatinterestinresearchdirectedtothe development of erbium-doped silica-based glasses obtained by the sol±gel route [1,2]. Silica-based sol-gelglassesactivatedbyEr 3 ionsareattractive materials for integrated optics devices such as frequency up converters and optical ampli®ers operating in the 1.5 lm band [1,2]. The main de- terminants of the performance of sol±gel-based devices are the non-radiative relaxation channels due to rare-earth RE) concentration quenching andvibrationsoftheOHgroups.InintegratedEr ampli®ers, few centimeters of optical path are needed,andtheerbiumconcentrationshouldbeas high as possible to obtain good optical ampli®ca- tion gain [1]. However, at increasing Er concen- tration, non-radiative energy transfer between neighbouring ions attributed to clustering of the RE ions) may take place and lead to both linear andnon-linearquenching[2].Earlierspectroscopic studieshaveshownthattheconcentrationandthe properties of the optically active ion aect the densi®cationandtheopticalpropertiesofthe®nal glass [2±5]. Alumina addition was found to im- prove the dispersion of the optically active ion in silica glasses prepared by sol-gel route [1,4,6]. Journal of Non-Crystalline Solids 293±295 2001) 112±117 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +39-0461 881 685; fax: +39- 0461 881 680. E-mail address: rocca@science.unitn.it F. Rocca). 0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0022-309301)00660-3