Nature of the boson peak in Raman spectra of sodium borate glass systems: influence of structural and chemical fluctuations and intermolecular interactions

Abstract

The nature of the boson peak in the low-wavenumber Raman spectra of (Na2O)x–(B2O3)1−x glasses is analyzed to be a manifestation of intermolecular interactions within regions of inhomogeneity, which are inherent to all glasses. The wavenumber of the boson peak satisfies the modified Szigeti and Odelevskii equation, previously successfully applied to studies of intermolecular vibrations in organic liquids and their mixtures. This allows one to conclude that the boson peak is connected with intermolecular interactions of structural groups, which are not bound in a glass-forming network, but form regions of structural and chemical inhomogeneities. The existence of such regions in alkali metal borate glasses and their high concentration were previously confirmed by small-angle x-ray scattering data. Thus intermolecular interactions can be described by a soft potential. It has been also found that the absolute intensity of the boson peak substantially decreases with an increase in the modifier concentration in accordance with the specific heat and the thermal phonon conductivity behavior as well as the theoretical expectations for glasses with increasing fragility. However, this decrease does not have a monotonic character but exhibits a slight bump at x = 0.2. Such a concentration behavior is connected with a decrease in the free volume in the vicinity of this concentration and a suppression of the relaxation processes in favor of vibrational modes, which begin to contribute to the boson peak. Copyright © 2000 John Wiley & Sons, Ltd.