Structural influence of PbO by means of FTIR and acoustics on calcium alumino-borosilicate glass system

Glasses of the system xPbO–RNa2B4O7–(100 − R − x)CAS with 0 ⩽ x ⩽ 50, and 50 ⩽ R ⩽ 75 mol% have been synthesized in two compositional series and characterized by FTIR, and acoustics to explore the role of PbO on the structure of the glasses. The elastic properties of glasses have been investigated using sound velocity measurements at 4 MHz. According to the IR analysis, there is a depolymerization of the silicate network and a conversion of BO3 into BO4 structural units with the formation of non-bridging oxygens. PbO is incorporated into borosilicate network via B–O–Pb, and Si–O–Pb bonds. It is assumed that PbO plays the role of a modifier by replacing the high strength B–O bond with a lower strength Pb–O bond which decreases the glass-transition temperature Tg. The increase of the density was attributed to the replacement of less dense borosilicate structural units by the more dense lead structural units. The observed compositional dependence of the ultrasonic velocity and the elastic moduli was interpreted in terms of the effect of PbO on the coordination of the borate structural units with oxygens and the depolymerization of the silicate network.