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 for more complex silicate glasses (binary, ternary etc...).In the first part of this talk, we will present the structural and vibrational properties a sodium silicateglass of composition 4Na 2 O-6SiO 2 (NS1.5). The study has been carried out using first principlescalculations within the density functional theory framework as implemented in the VASP code. Wehave studied the structural and vibrational properties of this glass, and we have identified thecontributions of the various species to the vibrational density of states (VDOS) as well as to the IRspectra. In particular we will discuss how the presences of the sodium atoms affect the relevantvibrational parameters: positions, shapes and intensities of the main peaks of the VDOS and IRspectra.In the second part, we will discuss the properties of several complex borosilicate glasses.Computing physical properties of complex glass compositions via numerical simulations oftensuffers of inaccurate predictions when using classical potentials, or it is excessively timeconsuming simulations when glass structures are obtained using ab-initio methods. The study hasbeen carried out by combining classical and ab initio molecular dynamics (MD) methods and byvarying the concentrations of SiO 2 , B 2 O 3 , Na 2 O, CaO, Al 2 O 3 and MgO. The classical MD includedliquid equilibration, quenching and preliminary relaxation, while ab initio method was used tofurther relax the samples. Two sets of effective potentials have been used for the classicalsimulations. This strategy allows of course a significant CPU time saving.We have compared the structural properties of the simulated samples and have found thatclassical MD simulations cannot provide reliable glass models for complex borosilicates (at leastwith the potentials we have chosed), especially for the peculiar aspect of Boron coordination. Theab initio simulations have corrected the unreliable classical simulated structures and providedresults in good agreement with previous studies of borosilicate glasses. We have also noticed thatthe presence of Al and Mg has influenced the fraction of 3-fold coordinated Boron, as expectedfrom experimental data. The vibrational properties have been equally studied within the ab initioapproach, and the contributions of the various species have been identified. We have found that 3-and 4-fold coordinated borons give rise to distinguished spectral features.

We have performed classical molecular dynamics simulations in order to study the changes under compression in the local and medium range structural properties of three sodium borosilicate glasses with varying sodium content. These glasses have been isostatically compressed up to 20 GPa and then decompressed in order to analyze the different mechanisms that affect densification, alongside with the permanent modifications of the structure after a full compression/decompression cycle. The results show that the atomic packing is the prominent characteristic that governs the amount of densification in the glass, as well as the setup of the permanent densification. During compression, the bulk modulus increases linearly up to approximately 15 GPa and more rapidly for higher pressures, a behavior which is reflected on the rate of increase of the average coordination for B and Na. Radial distribution functions at different pressures during the cycle help to quantify the amount of distortions in the elementary structural units, with a pronounced shortening of the Na–Na and Na–O bond lengths during compression. A subsequent decomposition of the glassy matrix into elementary Voronoi volumes verifies the high compressibility of Na-rich regions.

We use ab initio simulations to investigate the properties of a sodium borosilicate glass of composition 3Na2 O-B2 O3 -6SiO2 . We find that the broadening of the first peak in the radial distribution functions gBO (r) and gBNa (r) is due to the presence of trigonal and tetrahedral boron units as well as to nonbridging oxygen atoms connected to BO3 units. In agreement with experimental results, we find that the [3] B units involve a significant number of nonbridging oxygens, whereas the vast majority of [4] B have only bridging oxygens. We determine the three-dimensional distribution of the Na atoms around the [3] B and [4] B units and use this information to explain why the sodium atoms associated with the latter share more oxygen atoms with the central boron atoms than the former units. From the distribution of the electrons we calculate the total electronic density of states, as well its decomposition into angular momentum contributions. The vibrational density of states shows at high frequencies a band that originates from the motion of the boron atoms. We find that the [3] B and [4] B units give rise to well-defined features in the spectrum, which thus can be used to estimate the concentration of these structural entities. The contribution of [3] B can be decomposed further into symmetric and asymmetric parts that can also be easily identified in the spectrum. Furthermore, it is found that certain features in the spectrum can be used to obtain information on the type of atom that is the second-nearest neighbor of a boron in the [4] B unit. We calculate the average Born charges on the bridging and nonbridging oxygen atoms and show that these depend linearly on the angle between the two bonds and the distance from the connected cation, respectively. Finally, we have determined the frequency dependence of the dielectric function, as well as the absorption spectra. The latter is in good quantitative agreement with the experimental data.

We use ab initio simulations to study the static and dynamic properties of a sodium borosilicate liquid with composition 3Na2 O–B2 O3 –6SiO2 , i.e., a system that is the basis of many glass-forming materials. In particular, we focus on the question how boron is embedded into the local structure of the silicate network liquid. From the partial structure factors we conclude that there is a weak nanoscale phase separation between silicon and boron and that the sodium atoms form channel-like structures as they have been found in previous studies of sodosilicate glass-formers. Our results for the x-ray and neutron structure factor show that this feature is basically not detectable in the former but should be visible in the latter as a small peak at small wave vectors. At high temperatures we find a high concentration of threefold coordinated boron atoms which decreases rapidly with decreasing T , whereas the number of fourfold coordinated boron atoms increases. Therefore, we conclude that at the experimental glass transition temperature most boron atoms will be fourfold coordinated. We show that the transformation of [3] B into [4] B with decreasing T is not just related to the diminution of nonbridging oxygen atoms as claimed in previous studies, but to a restructuring of the silicate matrix. The diffusion constants of the various elements show an Arrhenius behavior and we find that the one for boron has the same value as the one of oxygen and is significantly larger than the one of silicon. This shows that these two network-formers have rather different dynamical properties, a result that is also confirmed from the time dependence of the van Hove functions. Finally, we show that the coherent intermediate scattering function for the sodium atoms is very different from the incoherent one and that it tracks the one of the matrix atoms.

We have carried out ab initio or combined classical and ab initio molecular dynamics (MD)simulations in order to investigate the structural and vibrational properties of several borosilicateglasses. We have considered rather simple ternary compositions with varying SiO 2 , B 2 O 3 or Na 2 Oconcentrations, or more complex compositions containing equally CaO, Al 2 O 3 or MgO. The ab initiocalculations have been carried out within the density functional theory framework as implementedin the VASP code. The classical MD simulations were carried out using different effective pairpotentials.We have studied the local structure of the various structural units, and in particular we havefocused on the structures around the boron atoms and how these are embedded into the network.We have investigated how the Na atoms are distributed around the [3] B triangles and [4] Btetrahedra. Furthermore, we have found that the Na distribution associated to a BO 4 tetrahedron isdifferent from that corresponding to a SiO 4 tetrahedron in that the former gives rise to a distributionthat is significantly more structured.The vibrational properties have been equally studied within the ab initio approach, and we haveidentified the contributions of the various species as well as those of the local structural units. Wehave also calculated the dielectric function ε(ω) as well as the absorption spectra. The latter are ingood quantitative agreement with experimental data.The results obtained in this work confirm that the atomistic simulations, in particular the ab initioones, give access to a better understanding of complex borosilicate glasses since their structuraland vibrational properties can be extracted with a good accuracy and compare very well to experimantal data.

Les méthodes expérimentales de caractérisation de matériaux vitreux ont connuun fort développement. Cependant, l'interprétation des spectres expérimentaux(Raman, hyper-Raman, IR, RMN, etc...) s'avère souvent difficile en raison dudésordre structural qui leur est propre. La connaissance des verres et liquidessilicatés (SiO 2 +oxydes) est essentielle pour faire progresser nombre de domainesscientifiques et industriels tels que la chimie des verres et céramiques,l'électronique, les sciences de la terre ou bien le confinement des déchetsindustriels et nucléaires.Les progrès constants de la puissance des ordinateurs, ainsi que des codes desimulation atomistique, ont permis l'essor des modélisations numériques, qui ontdésormais pris une place prépondérante en science en général, et dans la sciencedes verres en particulier. Depuis le milieu des années 90, il est devenu égalementpossible de simuler des verres au moyen de méthodes ab initio et ceci bien sûrgrâce au développement continu des ordinateurs parallèles.Les études réalisées au moyen d'approches ab initio sont bien sûr les plus fiables,mais, du point de vue pratique, ont encore un coût numérique très élevé, mêmepour les super-ordinateurs les plus rapides. Pour palier cet inconvénient, il fautconsidérer la description alternative basée sur la connaissance d'un potentielinteratomique effectif. Cette dernière donne accès à un gain d'un facteur 1000 aumoins sur la taille des modèles, et également pour les longueurs des trajectoires.Après un rappel bref des principes de base des simulations atomistiquesclassiques et ab initio, j'illustrerai à travers quelques exemples que ces simulationspermettent d’approfondir nos connaissances sur l'organisation structurale desverres silicates à l’échelle locale et intermédiaire, ou bien d’établir dans certainscas des corrélations directes entre les arrangements structuraux et des propriétésspectroscopiques et mécaniques.

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.