The origin of nanostructuring in potassium niobiosilicate glasses by Raman and FTIR spectroscopy

The structure of potassium niobium silicate glasses in a wide compositional range has been studied by means of Raman and FTIR spectroscopy. The glasses spectra were compared to those of the KNbSi2O7 and K3Nb3O6Si2O7 polycrystalline samples, obtained by crystallization of glasses of the same composition. It was found that the structure of such glasses is formed by SiO4 tetrahedra and distorted NbO6 octahedra. The amount of highly distorted (edge-sharing, non-bridging oxygens) octahedra results essentially unchanged from the glass composition. By contrast, the fraction of octahedra with a lower distortion degree (corner-sharing, bridging oxygens) increases with the Nb2O5 content. Raman and FTIR investigations indicate that during long heat treatments at temperatures near Tg, in the 23K2O · 27Nb2O5 · 50SiO2 glass, a structural change occurs regarding the amorphous matrix with a decrease of the niobium octahedra distortion. This can be related to a segregation process producing niobium rich regions nanometric in size. In the first heat treatment (2 h) the glass remains amorphous while for more prolonged heat treatments, nanocrystals of an unidentified phase are formed. In the same time the changes of the amorphous matrix hinder further crystallization.