The vibrational properties of three sodosilicate glasses have been investigated in the framework of Density Functional Theory. The Raman spectra are calculated and the results compare well with the experimental observations. The vibrational analysis confirms the presence of Si-O-Si bending as well as breathing modes of small rings at intermediate frequencies. The Qn Raman-feature at high frequency is decomposed into Q2, Q3, and Q4 vibrational species. Interestingly, the results suggests a large overlap between the three contributions as well as spectral shapes that are not necessarily unimodal nor Gaussian, as frequently assumed in the treatment of the experimental data. Raman scattering of ternary alumino-silicate glasses with alkali and alkaline-earth cations (Mg, Ca, Sr, Ba, Na,...) has also been performed. In these glasses, vibrational signatures of cations are clearly evidenced at low frequency in the depolarized (VH) spectra. Raman scattering by cations at low-frequency is confirmed by the computational data in the sodo-silicates. In the alumino-silicate glasses the responses associate to different types of motions. The analysis allows separating the contribution arising from network modifier cations to that originating from charge compensator ones.

We use computer simulations to probe the thermodynamic and dynamic properties of a glass-former that undergoes an ideal glass-transition because ofthe presence of randomly pinned particles.We find that even deep in the equilibrium glass state the system relaxes to some extent because ofthe presence of localized excitations that allow the systemto access different inherent structures,giving thus rise to a non-trivial contribution to the entropy. By calculating with high accuracy thevibrational part of the entropy, we show that also in the equilibrium glass state thermodynamicsand dynamics give a coherent picture and that glasses shouldnot be seen as a disordered solid inwhich the particles undergo just vibrational motion but instead as a system with a highly nonlinearinternal dynamics.

We propose a new scheme to parametrize effective pair potentials that can be used tosimulate oxide glasses. As input data for the optimization we use the radial distributionfunctions of the liquid and the vibrational density of state of the glass, both obtained fromab initio simulations, as well as experimental data on the pressure and/or composition de-pendence of the density and the elastic moduli of the glass [1].For the case of silica we find that this new scheme allows to find potentials that are signifi-cantly accurate than previous ones even if the functional form is the same, thus demonstratingthat even simple two-body potentials can be superior to more complex three-body poten-tials. We have tested the new potential by calculating the pressure dependence of the elasticmoduli and find a good agreement with the corresponding experimental data.For binary alkali (lithium, sodium, potassium) silicate glasses, the new potentials allowto reproduce the composition dependence of both density and elastic moduli. Further, weexamine the capabilities of these potentials for studying ternary compositions containing twoalkali oxides, and we find that they are reliable even if they have been developed for binarycompositions.Simplicity of the functional form also makes these potentials computationally more efficientthan potentials with more complex functional forms, and hence more suitable for simulationsinvolving large length and/or time scales scales.S. Sundararaman, L. Huang, S. Ispas, and W. Kob, ”New optimization scheme to obtaininteraction potentials for oxide glasses”, submitted (2018)

Upon mechanical loading, granular materials yield and undergo plastic deformation. The nature of plastic deformation is essential for the development of the macroscopic constitutive models and the understanding of shear band formation. However, we still do not fully understand the microscopic nature of plastic deformation in disordered granular materials. Here we used synchrotron X-ray tomography technique to track the structural evolutions of three-dimensional granular materials under shear. We establish that highly distorted coplanar tetrahedra are the structural defects responsible for microscopic plasticity in disordered granular packings. The elementary plastic events occur through flip events which correspond to a neighbor switching process among these coplanar tetrahedra (or equivalently as the rotation motion of 4-ring disclinations). These events are discrete in space and possess specific orientations with the principal stress direction.