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

Pseudo-binary sodium borate glasses containing (1 - y)Na₂B₄O₇-yM_aO_b (where M_aO_b = 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 M_aO_b. 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.