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

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 gla sses. The wavenumber of the boson peak satisfies the modified Szigeti and O delevskii equation, previously successfully applied to studies of intermole cular 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 f orm regions of structural and chemical inhomogeneities. The existence of su ch regions in alkali metal berate glasses and their high concentration were previously confirmed by small-angle x-ray scattering data. Thus intermolec ular interactions can be described by a soft potential, It has been also fo und that the absolute intensity of the boson peak substantially decreases w ith an increase in the modifier concentration in accordance with the specif ic heat and the thermal phonon conductivity behavior as well as the theoret ical expectations for glasses with increasing fragility. However, this decr ease 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 rela xation processes in favor of vibrational modes, which begin to contribute t o the boson peak. Copyright (C) 2000 John Wiley & Sons, Ltd.