THE BOSON PEAK IN ALKALI SILICATE-GLASSES

In an effort to understand the nature of the vibrational modes contrib uting to the Boson peak in glasses, we have performed a low frequency Raman study of different alkali silicate glasses at 300 K (X2O(g). SiO 2(1-g) = 25%, 35% by weight; X: Na, K, Rb, Cs). The introduction of al kali ions in silica is known to break Si-O bonds, resulting in non-bri dging oxygens and thus reducing the connectivity of the glass. Because of their identical charge, different alkali ions are expected to modi fy the glass network in the same way. We find that the shape of the Bo son peak can be well fitted with a Lorentzian squared function charact erized by a single parameter nu(0). Analysis of the dependence of nu(0 ) on the type and concentration of alkali suggests that the position o f the Boson peak is essentially governed by the ratio 'force constant' over 'mass' of localized oscillators and is not determined by the con nectivity of the glass. The results also show little difference betwee n quenched and annealed glasses, indicating the absence of a strong re lation to the extended structure of the glass. (C) 1997 Elsevier Scien ce B.V.