We present a study of the structural and dynamical properties of two borosilicate glasses of compositions Na2O-B2O3-SiO2 and Na2O-CaO-B2O3-SiO2, respectively. These compositions are thus similar to the glass wool that is used in our daily life. The study was carried out using first principles molecular dynamics simulations within the DFT framework as implemented in the VASP code. The samples contained about 300 atoms, which is rather demanding for ab initio calculations, but this size is necessary to the study of the intermediate order in glasses. In particular, we have investigated how boron atoms integrate and modify the silica network, how the presence of Na and Ca atoms affects the network connectivity We also investigate the correlations between the local environments of boron and oxygen atoms, and their atomic charges extracted from electron density based population analysis methods. Furthermore we have determined the vibrational properties and identified the contributions of the different species to the density of states. We find that 3- and 4-fold coordinated boron atoms give rise to specific features in the density of states. The infrared spectra of our glassy models is also presented and directly compared to experimental data.

Nous présentons une étude des propriétés structurales et dynamiques d'un verre borosilicaté de composition Na2O-B2O3-SiO2. Cette composition est similaire à celle de la laine de verre, utilisée dans notre vie quotidienne. L'étude a été réalisée à l'aide des simulations de dynamique moléculaire ab initio. Les échantillons numériques contiennent environ 300 atomes, ce qui est assez exigeant pour les calculs ab initio, mais cette taille est nécessaire pour l'étude de l'ordre intermédiaire dans ces verres. Nous avons étudié comment les atomes de bore intégrent et modifient le réseau silicaté, et comment la présence d'atomes de Na affecte la connectivité du réseau. En particulier, nos données montrent que, si la plupart des caractéristiques du sous-réseau silicaté ne présente pratiquement pas de dépendance en température, celles correspondant au sous-réseau borate présentent un comportement plus complexe, comme par exemple le rapport relatif entre les coordinences trigonale et tétraédrique. Les constantes de diffusion ainsi que les énergies d'activation ont été extraites, et nous avons constaté qu'elles sont dépendantes de l'espèce. Nous avons déterminé les propriétés vibrationnelles et identifié les contributions des différentes espèces à la densité d'états. Nous constatons que les bores 3 - et 4-coordonnés donnent lieu à des caractéristiques spécifiques dans la densité d'états vibrationnels. Nous présenterons également les spectres infrarouges de nos modèles vitreux, qui seront comparés aux données expérimentales existantes dans la littérature.

We present a study of structural and dynamical properties of a liquid and glassy sodium borosilicate of composition 3Na2O-B2O3-6SiO2, study carried out using first principles molecular dynamics simulations. This glass composition is thus similar to the glass wool that is used in our daily life. The simulations were carried out within the DFT framework as implemented in the VASP code. The samples contained 320 atoms, which is rather demanding for ab initio calculations, but it has allowed to study the intermediate order in glasses. In particular, we have investigated how boron atoms integrate and modify the silica network. Furthermore we have determined the vibrational properties and identified the contributions of the different species to the density of states. We found that 3- and 4-fold coordinated boron atoms give rise to specific features in the density of states. The infrared spectra of our glassy models is also presented and directly compared to experimental data.

We present a study of the structural and dynamical properties of liquid and glassy borosilicates of compositions Na2O-B2O3-SiO2 and Na2O-CaO-B2O3-SiO2. These compositions are thus similar to the glass wool that is used in our daily life. The study was carried out using first principles molecular dynamics simulations within the DFT framework as implemented in the VASP code. The samples contained about 300 atoms, which is rather demanding for ab initio calculations, but this size is necessary to the study of the intermediate order in glasses. In particular, we have investigated how boron atoms integrate and modify the silica network, how the presence of Na and Ca atoms affects the network connectivity, how their coordination, as well as the oxygen speciation are affected by the temperature. We also investigate the correlations between the local environments of boron and oxygen atoms, and their atomic charges extracted from electron density based population analysis methods. Furthermore we have determined the vibrational properties and identified the contributions of the different species to the density of states. We find that 3- and 4-fold coordinated boron atoms give rise to specific features in the density of states. The infrared spectra of our glassy models is also presented and directly compared to experimental data. N.B. This work was done in collaboration with Laurent Pedesseau (L2C and INSA Rennes) and Walter Kob (L2C).

The vibrational properties of silica glass have been intensively studied experimentally and theoretically during the last four decades. However, only few investigations have explored the evolution of the vibrational features of simple alkali glasses or silica under compression. In this contribution, we will present a mode analysis of two simple (Li/Na) alkali glass models obtained by combined classical and Car-Parrinello molecular dynamics. In particular we will discuss how the presence of the network modifier influences the relevant vibrational parameters: positions, shapes and intensities of the main peaks in the vibrational density of states (VDOS) and IR spectra. We will also show the decomposition of the VDOS on symmetry-adapted modes of the basic structural units of our models networks: SiO4 tetrahedra and SiOSi bridges. The same analysis will be carried out in the case of few silica glass models at normal density as well as at higher density generated by Car-Parrinello molecular dynamics simulations.

We have carried out first-principles molecular dynamics simulations of a sodium borosilicate (NBS) glass within the framework of DFT. The composition is 6SiO2-3Na2O-B2O3 and the density was fixed to 2.51g.cm-3. The system size is 320 atoms (i.e. 60 silicon, 180 oxygen, 60 sodium and 20 boron atoms) which is rather demanding for ab initio simulations and has been chosen to obtain a good statistics, in particular for quantities associated to the lowest concentrated element (boron). The simulations have been started at 4500K in the canonical ensemble (NVT) and followed by a simulation in the microcanonical ensemble (NVE). Subsequently, the system is progressively cooled to 300K within 60ps. In order to characterized the structure of the system we have calculated the pair correlation functions, the angular distributions, the Qn distributions, the non bridging oxygen's concentration as well as the partial structure factors. Moreover, we have studied the environment of 3- and 4-fold (BIII and BIV) coordinated boron atoms. Furthermore, the vibrational density of states has been studied in details. Indeed, both BIII and BIV atoms have specific frequencies, and the partial vibrational density of the 3-fold coordinated B atoms has been found to be a weighted sum of two specific contributions: 3-fold symmetric coordinated B (B ) atoms and asymmetric coordinated B (B ) atoms. Identically, the partial vibrational density of the 4-fold coordinated B atoms has been found to depend on the nature of their second neighbor respectively with 4 Si and 0 B atoms as second neighbor and so on. The infrared spectra of our glassy models is also presented.

Il s'agit d'une intervention illustrant la modélisation des propriétés vibrationnelles des verres structuraux. Elle a été dispensée dans le cadre d'une école thématique CNRS organisée par le GDR Verres, afin de permettre aux différentes équipes participantes soit de s'initier aux méthodes de modélisation, soit de se perfectionner, ou encore de comparer les démarches propres à celles développées dans d’autres sites ou communautés de recherche.