Résumé
The vibrational properties of silica glass have been intensively studied experimentally and theoretically during the last fourdecades. However there are few theoretical studies of the evolution of the vibrational properties of more complex silicateglasses (binary, ternary etc...).We have used first principles simulations in order to investigate the properties of two silicate glasses: a sodium silicate ofcomposition 4Na2O-6SiO2 (NS1.5) and a sodium borosilicate of composition 3Na2O-B2O3-6SiO2 (NBS). The studies werecarried up using first principles molecular dynamics within the density functional theory framework as implemented in theVASP code [1]. We have studied the structural and vibrational properties of these two glasses, and we have identified thecontributions of the various species to the vibrational density of states (VDOS) as well as to the IR spectra.In particular we will discuss how the presences of the sodium and boron atoms affect the relevant vibrational parameters:positions, shapes and intensities of the main peaks of the VDOS and IR spectra. We have found that 3- and 4-foldcoordinated boron atoms give rise to distinguished spectral features. Moreover, the partial vibrational density of the 3-foldcoordinated B atoms has been found to be a weighted sum of 2 specific contributions so-called 3-fold symmetriccoordinated B atoms and asymmetric coordinated B atoms.[1]. Kresse & Hafner (1993) PRB 47, 558; Kresse & Furthmüller (1996) Comp.Mater. Sci. 6, 15; Kresse &Furthmüller (1996) PRB 54, 1116