Influence of Ba2+ ions on defect concentration in bismuth silicate glasses evidenced by FTIR and UV-visible spectroscopy

Abstract

Bismuth silicate glasses containing Ba2+ ions were prepared by a normal melt quench technique in a wide range of composition. The present study includes structural and optical characterization of these glasses by using the techniques of IR and UV-visible spectroscopy. The variation in density and molar volume with Ba2+ ion concentration was related to structural changes occurring in the glass. The large difference in crystalline temperature (TC) and glass transition temperature (Tg) value shows the thermal stability of these glasses. Analysis of IR spectra shows the occurrence of broad and strong band around 456-471 cm-1, attributed to both Bi-O- stretching vibrations in distorted linked [BiO6] units and bending vibrations of the silicate network. The shifting of center of the band around 800 - 1100 cm-1 with the increase in Ba2+ ions concentration in glass matrix suggests a decrease in Si-O-Si bond angle and causes glass network shrinkage which is also supported by observed decrease in molar volume. The analysis of optical absorption spectra of these glasses reveals that the position of absorption edge and cutoff wavelength shift towards blue as Ba2+ ion concentration increases in the glass composition. The variation in optical band gap energy (Eg) with change in glass composition may be attributed to the indirect influence of the Ba2+ ions on the band gap and can be evaluated from the Tauc's plots for indirect allowed and indirect forbidden transitions. The increase in optical band gap (Eg) from optical absorption analysis indicates that the glass network becomes more compact and degree of disorder decreases with increase of concentration of Ba2+ ions. The origin of Urbach's energy is associated with phonon-assisted indirect transitions and its value varies with the random internal electric field either due to lack of long range order or presence of defects. From Urbach energy values, it was concluded that the defect concentration could be controlled by the presence of Ba2+ ions in the present glass system. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.