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).

The dynamics of glass forming liquids shows a tremendous slowing down if temperature is decreased: Relaxation times, transport coefficients such as the diffusion constant or the shear viscosity grow by more than 15 orders of magnitude within narrow temperature range. One of the central goals in the field of glass physics is to understand the origin of this dramatic slowing down of the dynamics.There are several scenarios that seem to be able to explain the slowing down of the dynamics and the low temperature state of the glass former. A very popular one is to rationalize the slow dynamics by invoking the existence of a thermodynamic transition, the so-called ideal glass transition from the liquid to an ideal glass states. This ideal glass transition occurs when the configurational entropy, defined as the logarithm of the number of available states, becomes zero. Due to thissingular behavior of the configurational entropy, the relaxation time andtransport coefficients diverge if the transition point is approached [1].However, confirming the existence of the ideal glass transition is a verydifficult task since in practice the system falls out of equilibriumbefore reaching this putative ideal glass transition temperature,thus making the test of this theory most difficult. Furthermore, otherscenarios are also able to explain the slow dynamics without resortingany thermodynamic transition [2].In the present study, we investigate the existence of the idealglass transition by using computer simulation of a canonical binaryLennard-Jones glass former in three dimensions. Massive computationaleffort and efficient sampling algorithm (parallel tempering) allow us toreach equilibrium states that are very deeply supercooled. By measuringthe configurational entropy directly (obtained via thermodynamicintegration), we find that the configuration entropy does not go to zeroat a finite temperature. This implies that the thermodynamic singularityis indeed avoided before reaching a putative ideal glass transitiontemperature predicted by previous studies. Our results indicates thatcontrary to the previous results the system remains in a liquid statedown to zero temperature without showing an ideal glass transition.Furthermore, we analyze the microscopic structure and the potentialenergy landscape of the system to clarify the mechanism that leads tothe avoidance of the ideal glass transition. We find that locallyfavored structures present in the liquid state hardly change in thetemperature range we consider, which rules out another possibility forthe slow dynamics, e.g. the transformation between two distinct liquids,so-called liquid-liquid transition [3]. Instead, we find the potentialenergy landscape contains extensive number of minima even very lowtemperature, which rationalize the avoided singularity.Our results support a theoretical argument that the ideal glasstransition can not exist at bulk system in finite dimensions [5].[1] Adam G and Gibbs JH, J. Chem. Phys., 43, 139 (1965)[2] Chandler D and Garrahan JP, Annu. Rev. Phys. Chem., 61, 191 (2010)[3] Speck T, Royall CP, and Williams SR, arXiv:1409.0751, (2014)[4] Saksaengwijit A and Heuer A, Phys. Rev. Lett., 93, 235701 (2004)[5] Stillinger FH, J. Chem. Phys., 88, 7818 (1988)

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.