A quaternary glass system Na1.4B2.8SixPb0.3-xO5.2+x\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Na}_{1.4}\hbox {B}_{2.8}\hbox {Si}_{x}\hbox {Pb}_{0.3-x}\hbox {O}_{5.2+x}$$\end{document}, with 0 ≤\documentclass[12pt]{minimal}
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\begin{document}$$\le $$\end{document} x ≤\documentclass[12pt]{minimal}
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\begin{document}$$\le $$\end{document} 0.3, was prepared and studied by Fourier transform infrared spectroscopy, density and ultrasonic techniques to debate the issue of the role of SiO2\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {SiO}_{2}$$\end{document} in the structure of lead alkali borate glasses. The results indicate that SiO2\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {SiO}_{2}$$\end{document} generates an abundance of bridging oxygen atoms, [BO4\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {BO}_{4}$$\end{document}] and [SiO4\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {SiO}_{4}$$\end{document}] structural units and changes the bonds B–O–B and Pb–O–B to Si–O–Si and B–O–Si. The latter bonds have higher bond strength and higher average force constant than the former bonds. Therefore, the glass structure becomes contracted and compacted, which decreases its molar volume and increases its rigidity. This concept was asserted from the increase in the ultrasonic velocity, Debye temperature and elastic moduli with the increase of SiO2\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {SiO}_{2}$$\end{document} content. The present compositional dependence of the elastic moduli was interpreted in terms of the electron–phonon anharmonic interactions and the polarization of Si4+\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Si}^{4+}$$\end{document} cation. A good correlation was observed between the experimentally determined elastic moduli and those computed according to the Makishima–Mackenzie model.
