Abstract
Glasses in the system (x/2)Bi(2)O(3)-(x/2)Nb(2)O(5)-(1 - x)Na(2)B(4)O(7), 0 <= x <= 0.3, have been prepared by the melt quenching technique. Elastic properties and Debye temperature (theta(D)) 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 Bi(2)O(3), and Nb(2)O(5) on the structure of Na(2)B(4)O(7) 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 Bi(2)O(3), and Nb(2)O(5) act as a network modifier in the range 0 <= x <= 0.15, while beyond x=0.15, Nb(2)O(5) acts as a network former which affects the cliborate units that mainly consist the strong borate network. These results are interpreted in terms of the transformation of NbO(6) into NbO(4), 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 Bi(2)O(3), and Nb(2)O(5) on the Boron-coord i nation number of the glass structure and to the relatively large electron-phonon anharmonic interactions. (C) 2008 Elsevier B.V. All rights reserved.