Structural and Dielectric Studies on Na 2 O-PbO-SiO 2 Glasses

Structural studies on x Na 2 O.(50- x )PbO.50SiO 2 glasses have been carried out by different spectroscopic techniques. FTIR, EDX, NMR and dielectric spectroscopies are applied to follow the change in glass structure in terms of bridging oxygen (BO), non-bridging oxygen (NBO), and cluster species upon adding of Na 2 O at expense of PbO. NBOs are formed in the silicate network upon Na 2 O addition, since Na 2 O plays the role of glass modifier. Changes in relative area of Q 2 (obtained from IR analysis) and NMR chemical shift of silicon nuclei with increasing Na 2 O are indicative for formation of the less shielded silicate units. While in sodium rich silicate glasses, the structural role of Na 2 O is changed due to changing Na coordination. FTIR analysis could be used to elucidate the changes related to changing the role of Na 2 O. The data obtained are correlated with that obtained from EDX spectroscopy. Moreover, the role of Na 2 O on the process of cluster formation when it substitutes PbO is determined. NBOs only are formed in the silicate network upon Na 2 O addition up to 30 mol%. On the other hand, in sodium rich silicate glasses an additional type of oxygen (free oxygen O 2− ) is present. The free oxygen is required for sodium to form aggregated cluster, specially at high concentration of Na 2 O (50 mol%). An increase in Na 2 O concentration in Na 2 O-rich silicate network results in increasing Na coordination instead of breaking more silicon–oxygen bonds. Scanning electron micrographs (SEM) and EDS spectroscopy in correlation with FTIR results confirm the presence of Na cluster species. The effect of clusters formation on AC conductivity was discussed according to jump relaxation model. Some parameters related to AC conductivity are found to be affected by the presence of cluster species in the glass network.