29Si MAS NMR and microhardness studies of some lead silicate glasses with and without modifiers

Lead silicate glasses having different mole ratios of SiO2 to PbO and containing different amounts of alkali/alkaline earth metal oxides were prepared by conventional melt-quench method. The different structural units of Si present in these glasses were identified by 29Si MAS NMR. Addition of PbO above 50 mol% to SiO2 and Na2O up to 15 mol% in (SiO2)0.5(PbO)0.5 glasses has been found to give similar 29Si MAS NMR patterns, characteristic of depolymerised silicon structural units (Q2 and Q1), which interconnect the extended Pb–O–Pb network in the glass. The microhardness values of lead silicate glasses show systematic decrease with increase in lead oxide contents. The microhardness values of lead silicate glasses having SiO2 to PbO mole ratio 6.9 and containing different amounts of alkali/alkaline earth metal oxides, have been found to vary in a complex manner. For higher concentrations of alkali/alkaline earth metal oxides, microhardness values have been found to decrease, probably due to conversion of more rigid covalent Q4, and Q3 structural units to less rigid Q2 and Q1 structural units.