Structural investigations of sodium diborate glasses containing PbO, Bi₂O₃ and TeO₂: elastic property measurements and spectroscopic studies

Abstract

Pseudo-binary sodium borate glasses containing (1 - y)Na₂B₄O₇-yMaOb (where MaOb = PbO, Bi₂O₃ and TeO₂) (y = 0.25, 0.5, 0.67 and 0.79) have been investigated. Sound velocities (longitudinal and shear) have been measured at 10 MHz frequency using quartz transducers. Density increases with increase of y and the molar volume decreases. Sound velocities also decrease with increasing y till y ~ 0.66 above which it increases slightly. Steeper decrease in velocities has been observed in TeO2 containing glasses. Elastic moduli, Poisson's ratio and Debye temperature have been calculated. Glass transition temperatures have also been determined and it decreases with increase of y. T_g also exhibits a dependence on the cationic charge in MaOb. Infrared spectra of the glasses reveal that the strong network consisting of diborate units is affected only by PbO and only very marginally by Bi₂O₃ and TeO₂. Only glasses with high concentrations of Bi₂O₃ and TeO₂ reveal the presence of mixed bridges such as Bi-O-B and Te-O-B. Consistent with the IR spectral observations, the N₄ values of ¹¹B MAS-NMR remain close to the ideal value of 0.5 of the diborate composition in most of the glasses. A structural model based on the observation that the diborate network is unaffected by Bi₂O₃ and TeO₂ where as PbO opens up and breaks the diborate units is shown to be consistent with all of the experimental observations including mechanical properties.