Structural aspects of B2O3-substituted (PbO)0.5(SiO2)0.5 glasses

Abstract

Lead borosilicate glasses having general formulae (PbO)0.5−x(SiO2)0.5(B2O3)x with 0.0 ≤ x ≤ 0.4 and(PbO)0.5(SiO2)0.5−y(B2O3)y with0.0 ≤ y ≤0.5 have been prepared by a conventional melt–quenchmethod and characterized by29 Si,11B magic angle spinning (MAS) NMR techniques and infraredspectroscopy, as regards their structural features. From29Si NMR results, it has been inferred that with increasing concentration ofboron oxide, (PbO)0.5−x(SiO2)0.5(B2O3)x glasses exhibit a systematicincrease in the number of Q4 structural units of Si at theexpense of Q2 structural units, along with the formationof Si–O–B linkages. On the other hand, for(PbO)0.5(SiO2)0.5−y(B2O3)y glasses, there is no directinteraction between SiO2and B2O3 inthe glass network, as revealed by the29 SiMAS NMR studies. Boron exists in both trigonal and tetrahedral configurations forthese two series of glasses and for the(PbO)0.5(SiO2)0.5−y(B2O3)y series of glasses;the relative concentration of these two structural units remainsalmost constant with increasing B2O3concentration. In contrast, for(PbO)0.5−x(SiO2)0.5(B2O3)x glasses,there is a slight increase in the number of BO3 structuralunits above x = 0.2,as there is a competition between SiO2 andB2O3 for interaction with Pb2+,thereby leading to the formation of BO3 structural units.For both series of glasses, the thermal expansion coefficient isfound to decrease with increasing B2O3concentration, the effect being more pronounced for the (PbO)0.5−x(SiO2)0.5(B2O3)x series of glasses due to the increased concentration ofQ4structural units of silicon and better cross-linking asa result of the formation of Si–O–B-type linkages.