Cation dependence of raman scattering in alkali borate glasses

Abstract

A Raman scattering study of alkali borate glasses in the system xR 2 O · (1 − x) B 2 O 3 for R = Li, Na, K has been carried out to understand the role of the mass of the cation in determining the B-O network in glasses. Raman results show that lighter cations favour a stronger B-O network in which three-dimensional BO 4 groups are attached to a large borate ring while the heavy cations are not easily inserted in the network interstices. The introduction of heavy cations favours the formation of non-bridging oxygens which make the B-O network comparatively weak and hence result in a decrease of the glass transition temperature ( T g). The short correlation range in these glasses for different R + has also been estimated and its dependence on the mass of R + is discussed in terms of a coordination change of boron from three to four. The frequency and temperature-reduced intensities have been calculated for these glasses, and it has been found that the temperature-reduced Raman spectra represent the true vibrational density-of-states. This result also corresponds well to theoretical results obtained on the basis of the Martin-Brenig model. A band around 770 cm −1 whose position varies with changes in both the composition and the alkali ion is explained by considering a simple model based on two weakly coupled harmonic oscillators.