First-principles modelling of silicate glasses: interplay between structural and vibrational properties
Résumé
The vibrational properties of silica glass have been intensively studied experimentally and theoretically during the last four decades. However there are few theoretical studies of the evolution of the vibrational properties of more complex silicate glasses (binary, ternary etc...).
We have used first principles simulations in order to investigate the properties of two silicate glasses: a sodium silicate of composition 4Na2O-6SiO2 (NS1.5) and a sodium borosilicate of composition 3Na2O-B2O3-6SiO2 (NBS). The studies were carried up using first principles molecular dynamics within the density functional theory framework as implemented in the VASP code. We have studied the structural and vibrational properties of these two glasses, and we have identified the contributions 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-fold coordinated boron atoms give rise to distinguished spectral features. Moreover, the partial vibrational density of the 3-fold coordinated B atoms has been found to be a weighted sum of 2 specific contributions so-called 3-fold symmetric coordinated B atoms and asymmetric coordinated B atoms.