Thermally induced segregation of SnO2 nanoclusters in Sn-doped silica glasses from oversaturated Sn-doped silica xerogels

Silica glasses with substitutional Sn-doping or with dispersion of SnO2 nanoclusters were prepared by the sol-gel method. The occurrence of SnO2 clustering was investigated by optical absorption, Raman scattering and electron paramagnetic resonance spectroscopies, varying both the tin concentration and the densification temperature of Sn-doped silica xerogels. The 3.7 eV absorption edge of SnO2 and the A(1g) Raman mode at 630 cm(-1) of the SnO2 phonon spectrum were used to assess the presence of SnO2 nanoclusters; the yield of X-ray induced paramagnetic E' Sn centres was monitored to probe the variation in the amount of substitutional Sn centres. The results indicate that the clustering of SnO2 becomes effective for Sn concentrations greater than or equal to 0.8 mol% and it is competitive with the substitutional doping in xerogels densified at T greater than or equal to 900 degreesC. No evidence of pre-existent clustering was observed in xerogels densified at lower temperatures in the investigated doping range.