We have parameterized empirical potentials for molecular dynamics (MD) simulations ofmulti-component silicate glasses. The main motivation has been to improve predictions ofstatic properties like elastic moduli and dynamic properties like vibrational density of states(VDOS) that MD simulations have generally not been able to estimate correctly, while stillusing a simple functional form for computational efficiency.Our approach has been to fit the potentials to data extracted from accurate first principlescalculations to predict both the static and dynamic properties correctly, by explicitly incor-porating the radial distribution function (RDF) and the VDOS into the cost function of thefitting scheme. The current optimization scheme is an extension of a recent work using asinput only the structural data from the ab initio simulations, and which has been applied inthe past to obtain a reliable potential for amorphous silica [1,2].The newly developed potentials will be used to study the elastic response of multi-componentoxide glasses to external stimuli such as high temperatures, high pressures and high strains,and their deformation modes under different loading conditions.A. Carré, J. Horbach, S. Ispas, W. Kob, Europhys. Lett. 82 (2008) 17001.A. Carré, S. Ispas, J. Horbach, W. Kob, Comput. Mater. Sci. 124 (2016) 323

Density functional theory is used to calculate the vibrational properties of pure silica and sodo-silicateglasses with 20, 25, and 33 mol% of Na2O. The infrared and Raman spectra are calculated and the full responses aredecomposed into principal structural components (PSC). Those are for example the SiO4n--tetrahedra with n nonbrigdingoxygens defining the Qn-species at the origin of the structured feature at high frequency, and the Si-O-Sibridges leading the broad Raman R-band at intermediate frequencies. Our results confirm that Si-O-Si bending inbridges with large angle vibrate preferentially at lower frequencies than those with low angle. In addition, the spectralresponse of the Q2-species is bimodal and overlaps with that of the Q3, while the Q4 response covers almost all ofthe spectral range of the Qn-band. The ab-initio individual spectral responses of the PSC are used to reconstruct theexperimental Raman responses. Contrary to the commonly used multi-Gaussian decomposition, this approachprovides unambiguous band-assignments and hence a more accurate way to probe the structural and chemicalproperties of glasses from their spectroscopic signature.

Polarized Raman scattering is performed in a series of sodo-silicate glasses. The Si–O–Si inter-tetrahedral angle and its distribution are extracted using a model developed previously for densified silica, and the result are compared to ab initio atomistic calculations. The two techniques reveal a reduction of the most probable angle of about 0.30°/mol% of Na2O, but the shape of the angular distributions are different. The results suggest that Raman scattering enhances a specific angular distribution of Si–O–Si bridges, likely those close to sodium atoms, highlighting local angular heterogeneities.

In this work we have carried out variable-load indentation testing on three sodium borosilicate glassesand characterized the indentation imprints using Raman spectroscopy. The resulting analysis of thespectra helps describe the relative amount of densification and shear ow appearing in each glass, as wellas the different microstructural mechanisms that take place during deformation. Additionally, we showthat the two glasses that are relatively poor in sodium exhibit enhanced mixing of the silicate and borateunits after indentation. For the indentations performed using the highest load, we also acquired spectraby focusing at various depths below the imprint, in order to characterize the deformed region of theglasses and estimate its size. Similar experiments have been also carried out for one of the three glassesafter neutron irradiation, in order to study the effects of mechanical deformation on its depolymerizedand swollen structure.

Raman spectroscopy is one of the key experimental methods for the structuralanalysis of glasses, thanks to its simple application and its sensitivity to variations ofcomposition or atomic-scale order. However, the inherent disorder of the glassystructure results to broad and overlapping peaks in the spectra, rendering theirquantitative analysis a challenging task and resulting in a mostly phenomenologicalinterpretation. This drawback can be addressed by the use of modern and accuratetheoretical tools, such as ab-initio vibrational spectroscopies. In this work we havestudied three simple sodosilicate glasses, prepared using a combined classical/ab-initio approach. For these structures, we have used density functional theory (DFT) tocalculate their vibrational density of states, as well as their Raman and IR spectra.The main advantage of the DFT calculations is that the resulting spectra can bedirectly correlated with the structural models and, most importantly, the vibrationalspectrum can be decomposed into exact contributions arising from individualstructural units. In the case of the sodosilicate glasses, we have identifiedcorrelations between spectral characteristics and stoichiometry, the effect of thepresence of bridging and non-bridging oxygens, the contributions of smaller or largerSi-O-Si angles, as well as the ones arising from the presence of the so-called Q-species. These results open the way for the study of even more complex glasses andaid in the interpretation of experimental spectra.

Silicate glasses possess a central role in glass technology due to their multiple applications ranging from optical devices to the immobilization of nuclear waste. Raman spectroscopy is a key method for the investigation of their structure and the study of their properties. However, the inherent structural disorder found in glasses results in very broad and overlapping peaks in their spectra. In this context, an accurate theoretical modeling of the 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 binary sodo-silicate glasses [xNa2O- (1-x)SiO2], which have been prepared by combining classical and ab-initio Molecular Dynamics. The Raman spectra (polarized and unpolarized), as well as the IR spectra, have been obtained using the Quantum Espresso package [1] in the GGA framework by employing norm-conserving pseudopotentials. Calibration runs have been carried out on the crystalline α-quartz and Na2SiO3 systems in order to optimize the choice of pseudopotentials. Concerning the glasses under study, we focused on the effect of local structural units, such as Q-species and their interconnection, alongside the role of sodium atom content in order to assign the corresponding bands. The initial results show very good agreement with previous theoretical and experimental spectra [2-5], while the ongoing decomposition relative to the exact contributions from each of the structural units of the glasses is expected to provide insight to the interpretation of the experimental spectra.References[1] P. Gianozzi et al., J. Phys.: Condens. Matter 21, 395502 (2009).[2] S. Ispas, N. Zotov, S. de Wispelaere, W. Kob, J. Non-Cryst. Solids 351, 1144 (2005).[3] N. Zotov, H. Keppler, Phys. Chem. Minerals 25, 259 (1998).[4] B. Hehlen, D.R. Neuville, J. Phys. Chem. B 119, 4093 (2015).[5] B.O. Mysen, J.D. Frantz, Contrib. Mineral. Petrol. 117, 1 (1994).