Physical and structural properties of some bismuth borate glasses

Glasses in the system (x/2)Bi2O3-(x/2)Nb2O5-(1 − x)Na2B4O7, 0 ≤ x ≤ 0.3, have been prepared by the melt quenching technique. Elastic properties and Debye temperature (θD) have been investigated using sound velocity measurements at 4 MHz. The ultrasonic parameters along with the IR spectroscopic studies have been employed to study the role of Bi2O3, and Nb2O5 on the structure of Na2B4O7 glass. The density, the molar volume, the sound velocities, and the glass transition temperatures of these samples have been found to be compositional dependent. The results indicate that Bi2O3, and Nb2O5 act as a network modifier in the range 0 ≤ x ≤ 0.15, while beyond x = 0.15, Nb2O5 acts as a network former which affects the diborate units that mainly consist the strong borate network. These results are interpreted in terms of the transformation of NbO6 into NbO4, the increase in the number of non-bridging oxygen atoms, and the substitution of longer bond lengths of Bi-O, and Nb-O in place of shorter B-O bond. The observed compositional dependence of the elastic moduli is interpreted in terms of the effect of Bi2O3, and Nb2O5 on the Boron-coordination number of the glass structure and to the relatively large electron-phonon anharmonic interactions.