In order to explore the structural mechanisms responsible for the plastic behaviour and polyamorphism in silica glass, we perform Molecular Dynamics (MD) simulations within the framework of Density Functionnal Tight-Binding (DFTB) approach [1]. The DFTB method provides a description of atomic interactions with an accuracy almost similar to standard ab initio methods used in materials science (e.g. Density Functional Theory), but with a computational loading considerably lower. Recently, the DFTB approach applied to compressed silica (SiO2) glasses has shown that the structural changes from low- to high-density amorphous structures occur through a sequence of percolation transitions [2]. These transitions also provide an explanation of some of the mechanical properties of silica glass such as the Bulk modulus. Thus, the presence of percolation networks in silica glass sheds light on amorphous-amorphous phase transitions. In the present work, we present a more in details analysis of the local structures, their connectivity and features at medium range order. Hence, we have studied how the conformation of various polyhedra does change under compression, as well as how the network connectivity evolves from corner-sharing to edge- and face-sharing. We have also investigated the rings distribution and their conformation under pressure. We also present some results obtained for decompressions starting at pressures above the elastic to plastic limit around 10 GPa. In particular, we have observed various hysteresis for the pressure dependence of the density, the structural properties and percolation transitions. [1] M. Elstner and G. Seifert. Density functional tight binding. Phil. Trans. R. Soc. A, 372, 2014.[2] A. Hasmy, S. Ispas, and B. Hehlen. Percolation transitions in compressed SiO2 glasses. Nature, 599,2021.

We have used large-scale molecular dynamics simulations in order to investigate the surface properties of lithium, sodium, and potassium silicate glasses containing 25 mol% of alkali oxide. We have studied two types of surfaces, a melt-formed surface (MS) and a fracture surface (FS), and we have found that the influence of the alkali modifier on the surface properties depends strongly on the nature of the surface. The MS show a saturation of elemental concentration when the alkali is changed from Na to K, while the FS exhibit a monotonic increase of modifier concentration with increasing the alkali size. For the FS we find that larger alkali ions reduce the concentration of under-coordinated Si atoms at the surface and increases the fraction of two-membered rings.For both types of surfaces, the surface roughness of the surfaces are found to increase with alkali size, with the effect being more pronounced for the FS than for the MS. The height-height correlation function of the surfaces show a scaling behavior that is independent of the alkali species considered: The one for the MS is compatible with the prediction of thefrozen capillary wave theory while the ones for the FS shows a logarithmic growth, i.e., on the nanoscale these surfaces are not self-affine fractals. The influence of the modifier on the surface properties are rationalized in terms of the interplay between multiple factors involving the size of the ions, bond strength, and charge balance on the surface.