In recent years, Raman spectroscopy has become a toolof choice for the structural analysis of glasses, due to its simplicity and thelow cost of acquisition. However, the broad and overlapping peaks observedin the spectra result in a phenomenological and often qualitativeinterpretation. In order to improve the analysis of Raman spectra, and to startto obtain a more quantitative interpretation, one needs to identify the exactcontributions arising from individual structural units.We have used first-principles and combined classical/first-principlesapproaches to create representative atomistic models of some simple binarysoda- and lime-silicates, as well as for some ternary aluminosilicates. Withinthe density functional theory framework, we have calculated the vibrationaldensity of states as well as the IR and Raman spectra of these glass models,and we have found a good agreement with the experimental spectra.The knowledge of the theoretical spectra and the atomic structure has madepossible to identify the signatures of the various constituents of the glasses inthe spectra, as for example the Qi species. The obtained correlations can bethen used to better assign the main bands present in the spectra of morecomplex silicate glasses.

We discuss two procedures to obtain empirical potentials from ab initio trajectories. The first methodconsists in adjusting the parameters of an empirical pair potential so that the radial distribution functionsextracted from classical simulations using this potential match the ones extracted from the ab initio sim-ulations. As a case study, we consider the example of amorphous silica, a material that is highly relevantin the field of glass science as well as in geology. With our approach we are able to obtain an empiricalpotential that gives a better description with respect to structural and thermodynamic properties thanthe potential proposed by van Beest, Kramer, and van Santen, and that has been very frequently usedas a model for amorphous silica. The second method is the so-called ‘‘force matching” approach proposedby Ercolessi and Adams to obtain an empirical potential. We demonstrate that for the case of silica thismethod does not yield a reliable potential and discuss the likely origin for this failure.

We have carried out classical molecular dynamics simulations in order to get insight into the atomistic mechanisms of the deformation during nanoindentation of the pristine and irradiated forms of a sodium borosilicate glass. In terms of the glass hardness, we have found that the primary factor affecting the decrease of hardness after irradiation is depolymerization rather than free volume, and we argue that this is a general trend applicable to other borosilicate glasses with similar compositions. We have analyzed the changes of the short- and medium-range structures under deformation and found that the creation of oxygen triclusters is an important mechanism in order to describe the deformation of highly polymerized borosilicate glasses and is essential in the understanding of the folding of large rings under stress. We have equally found that the less polymerized glasses present a higher amount of relative densification, while the analysis of bond-breaking during the nanoindentation has showed that shear flow is more likely to appear around sodium atoms. The results provided in this study can be proven to be useful in the interpretation of experimental results.

We have carried out Molecular Dynamics simulations on a sodium borosilicate glass in order to analyze how the structure of the glass during irradiation is affected by the choice of the density in the liquid state before cooling. In a pristine form generated through the usual melt-and-quench method, both short- and medium-range structures are affected by the compressive or tensile environment under which the glass model has been generated. Furthermore, Na-rich areas are much easier to compress, producing a more homogeneous glass, in terms of density, as we increase the confinement during the quench. When the glass is subjected to displacement cascades, the structural modifications saturate at a deposited energy of approximately 8 eV/atom. Swelling appears for the glasses that were initially prepared under compression, while contraction is evident for the ones prepared under tension. We have equally prepared glass models using a fast quench method, and we have found that they present an analogous disorder as the glasses submitted to displacement cascades. Compared to the irradiated glass, we found that the magnitude of the modifications for the fast quenched glass is lower, most notably in terms of boron and sodium coordination, the percentage of non-bridging oxygens and in the ring distributions. This later result agrees with statements extracted from recent experimental works on nuclear glasses.

Silicates possess a central role in glass technology due to their multiple applications ranging from optical devices to the immobilization of nuclear waste. In this context, an accurate theoretical modeling of their spectra can be proven to be invaluable in order to optimize their performance and tailor their fabrication method to match requirements for future applications. In this work, we present results on simulated Raman spectra of simple sodo-silicate glasses, which have been prepared by combining classical and Car-Parrinello Molecular Dynamics. We focus on the effect of local structural units, such as SiO4 tetrahedra and their interconnection, alongside the role of sodium atom content in order to assign the corresponding bands. The obtained information is then used in order to help interpret the experimental spectra obtained for more complex sodium-borosilicate glasses.

In this study we present the results of nanoindentation and isostatic compression simulations of sodium borosilicate glasses using Molecular Dynamics, alongside experimental results obtained by Raman spectroscopy. For nanoindentation, three glasses with varying sodium content were simulated both in their pristine form, as well as a “disordered” one, which is analogous to the real irradiated glass and exhibits a swollen and more depolymerized network. The results indicate a very low increase of coordination of the silicon atoms, in contrast to boron and sodium. Densification occurs by the decrease of free volume and takes place in regions rich in glass formers. On the contrary, we observed evidence of high shear flow around sodium atoms. In the compression simulations, the pristine forms of the glasses were brought to a pressure of 20 GPa and then decompressed again. We found that the amount of free volume has a major influence on the amount of densification, alongside the percentage of three-coordinated boron. At the same time, sodium creates soft regions in the glass that can be more easily compressed. The results are then compared to Raman spectra collected on the imprint marks of experimentally indented borosilicate glasses.

Silicate glasses possess a central role in glass technology due to their multipleapplications ranging from optical devices to the immobilization of nuclear waste. In this context,an accurate theoretical modeling of their spectra can be proven to be invaluable in order tooptimize their performance and tailor their fabrication method to match requirements for futureapplications.The vibrational properties of silica glass have been intensively studied experimentallyand theoretically during the last four decades. However there are few theoretical studies of theevolution of the vibrational properties under pressure. We have calculated the parallel andperpendicular Raman spectra of the silica glass, within the density functional theory framework.At zero pressure, we have found a good agreement with the experimental spectra as well as toprevious calculations reported in the literature. Modifications of the Raman spectra underpressure have been found to be in agreement with experimental data.We will equally present preliminary results on simulated Raman spectra of a binary sodo-silicate glass. We focus on the effect of local structural units, such as SiO 4 tetrahedra and theirinterconnection, alongside the role of sodium atom content in order to assign the correspondingbands. The obtained information can be then used in order to help to interpret the experimentalspectra obtained for more complex silicate glasses.