Site of transition metal ions in ion-exchanged metal-doped glasses

Metal-for-alkali ion-exchange is largely used to dope surface layer of glass with metal ions so inducing a modification of the optical properties of the doped layer, useful to fabricate low-loss optical waveguides. X-ray absorption spectroscopy is a particularly important technique used to investigate the site of the metal ions introduced into the matrix, in specific cases also singling out the dopant oxidation state. Two case studies are reported in this paper, namely Cu-for-Na and Ag-for-Na ion-exchange layers of silicate glasses. The experimental results show that the site of the dopant ion is different from the one that replaced alkali ion: while in the first case the Cu site (i.e. the first oxygen shell around the metal species) is a linear combination of the sites of the crystalline oxides (CuO or Cu2O), with linear combination coefficients that depend on the average oxidation state, in the case of Ag, the Ag-O distance is much longer than in the Ag2O. Moreover, it comes to depend critically on the dopant concentration and on the subsequent heating treatments, indicating a higher stability of the Ag sites that exhibit a shorter Ag-O distance.