The vibrational properties of silica glass have been intensively studied experimentally andtheoretically during the last four decades. However there are few theoretical studies of theevolution of the vibrational properties under pressure.We have calculated the parallel and perpendicular Raman spectra of the vitreous silica. The studyhas been carried out using first principles calculations within the density functional theoryframework. The calculated spectra at ambient pressure are in good agreement with theexperimental ones as well as with previous calculations reported in the literature. Themodifications of the Raman spectra under pressure also reproduce the experimental data. Apreliminary analysis of the resulting structural modifications and their correlations with the changesseen in the spectra will be reported.

The art of deriving classical potential consists in finding the correct balance between the complexity of the functional form whose implementation should be as simple as possible, the computational efficiency, and the reliabilty. In this talk, we will present a new methodology which uses ab initio data for parametrizing classical potentials suited for amorphous systems.We have considered a simple and widespread empirical Ansatz, i.e. Born-Mayer-Coulomb, with the goal to investigate large time/size scales. As a first condition, a classical potential should be able to describe the atomic structure of the system under investigation. In order to optimize the parameters of the classical potential we have then used a cost function that involves the partial pair correlation functions extracted from ab initio simulations. The system considered is liquid silica, a liquid which represents a prototype of strong glass formers exibiting a network structure. From a methodological point of view, we will show, through the different comparisons between ab initio and classical MD, that this structural fitting procedure is a very promising approach to obtain new effective potentials suited for describing not only amorphous material but also crystalline properties.We will present simulations carried out using this new potential and showing that CHIK reproduces with more accuracy a number of properties of molten and glass silica, than the well known BKS potential. Some tests performed on a-quartz also showed that the new potential can also be used to simulate crystalline systems with a good accuracy regarding properties such as asymmetric coordinates and elastic constants. Finally, we will discuss molecular dynamics simulations carried out to represent the structures of amorphous dehydroxylated silica surface in liquid and glass states, and show that the new potential gives a also fair description of the surface.

Dans cet exposé, nous présenterons une analyse des propriétés structurales et vibrationnelles de plusieurs verres silicatés. Ces données ont été obtenues soit au moyen de simulations de dynamique moléculaire ab initio soit en combinant des simulations classiques et ab initio. Les simulations ab initio ont été réalisées dans le cadre de la DFT, avec le code VASP. Nous avons considéré des compositions binaires soda-silicatés ainsi que des compositions borosilicatés plutôt complexes contenant 4 ou 5 oxydes. Les échantillons numériques vitreux ainsi obtenus présentent des caractéristiques structurales et vibrationnelles en très bon accord avec les données expérimentales. De plus, nous avons pu identifier des corrélations entre les unités structurales locales et leurs signatures vibrationnelles.

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

We have used first principles simulations in order to investigate the properties of a sodium borosilicate glass of composition 3Na2O-B2O3-6SiO2 (NBS). This composition is similar to that of the glass wool used in our daily life. The study was carried up using first principles molecular dynamics within the density functional theory framework as implemented in the VASP code [1]. In this talk, we will present the analysis of the local environments of the three building structural units of the glass network, namely silicon atoms in 4-fold coordination, and boron atoms with 3- or 4-fold coordination. We will also discuss the local distribution of the Na atoms around the basic structural units. Indeed we have identified their preferential neighborhoods and how the nature of network former and its coordination infer on the shape of these preferential regions of Na atoms. The vibrational properties have been equally studied, and the contributions of the various species have been identified. 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. [1] Kresse & Hafner (1993) PRB 47, 558; Kresse & Furthmüller (1996) Comp.Mater. Sci. 6, 15; Kresse & Furthmüller (1996) PRB 54, 1116