The Boson peak in alkali silicate glasses

Abstract

In an effort to understand the nature of the vibrational modes contributing to the Boson peak in glasses, we have performed a low frequency Raman study of different alkali silicate glasses at 300 K (X 2O ( g) ·SiO 2(1 - g ) = 25%, 35% by weight; X: Na, K, Rb, Cs). The introduction of alkali ions in silica is known to break SiO bonds, resulting in non-bridging oxygens and thus reducing the connectivity of the glass. Because of their identical charge, different alkali ions are expected to modify the glass network in the same way. We find that the shape of the Boson peak can be well fitted with a Lorentzian squared function characterized by a single parameter ν 0. Analysis of the dependence of ν 0 on the type and concentration of alkali suggests that the position of the Boson peak is essentially governed by the ratio ‘force constant’ over ‘mass’ of localized oscillators and is not determined by the connectivity of the glass. The results also show little difference between quenched and annealed glasses, indicating the absence of a strong relation to the extended structure of the glass.