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International audience; Proceedings of the Thirteenth European Powder Diffraction Conference This special issueofPowder Diffraction offers a selection of contributions presented at theEuropean Powder Diffraction Conference (EPDIC) held in Grenoble, France on October 28-31,2012 (EPDIC- 13). EPDIC is the only European conference completely dedicated to all aspects of the analysis of polycrystalline materials by diffraction methods. Started in Munich , Germany in 1991, EPDICs have rapidly become reference poi nts for all researchers in the field and are now regarded to be ideal venues for the presentation and diffusion of new developments in powder diffraction instrumentation, analysis, and applications. The guest editors of this special issue of Powder Diffraction Michela Brunelli, ILL, Grenoble, France Pierre Bordet, CNRS, Grenoble, France Paolo Scardi, University of Trento, Trento, Italy

The Ag site in Ag-for-Na ion-exchanged borosilicate and germanate waveguides has been investigated by EXAFS spectroscopy. In contrast with the results generally reported in literature, it is shown that, for both silicate and germanate waveguides, the Ag site is strongly dependent on the Ag content and the Ag-O distance is longer for higher doping levels. It is demonstrated that this trend is related to a structural rearrangement of the Ag site in the whole doped layer; the EXAFS results on the samples that underwent the ion exchange at higher temperatures and the comparison with literature data suggest that the sites with shorter distances are more stable. The correlation between local structure around Ag and the photoluminescence properties of the borosilicate waveguides is established. In particular, the red shift of the blue band observed in the absorption and excitation spectra by increasing the Ag concentration has been univocally related to the increase of the Ag-O bond length.