The medium-range order (MRO) in amorphous systems has been linked to complex features such as the dynamic heterogeneities in supercooled liquids or the plastic deformation of glasses. However, the nature of the MRO in these materials has remained elusive, primarily due to the lack of methods capable of characterizing this order. Here, we leverage standard two-body structural correlators and advanced many-body correlation functions to probe numerically the MRO in prototypical network glass formers, i.e., silica and sodium silicates, systems that are of great importance in natural as well as industrial settings. With increasing Na concentration, one finds that the local environment of Na becomes more structured and the spatial distribution of Na on intermediate length scales changes from blob-like to channel-like, indicating a growing inhomogeneity in the spatial Na arrangement. In parallel, we find that the Si-O network becomes increasingly depolymerized, resulting in a ring size distribution that broadens. The radius of gyration of the rings is well described by a power law with an exponent around 0.75, indicating that the rings are progressively more crumpled with increasing size. Using a recently proposed four-point correlation function, we reveal that the relative orientation of the tetrahedra shows a surprising transition at a distance around 4 Å, a structural modification that is not seen in standard two-point correlation functions. The order induced by this transition propagates to larger distances, thus affecting the structure on intermediate length scales. Furthermore, we find that, for Na-rich samples the length scale characterizing the MRO is nonmonotonic as a function of temperature, caused by the competition between energetic and entropic terms which makes that the sample forms complex mesoscpic domains. Finally, we demonstrate that the structural correlation lengths as obtained from the correlation functions that quantify the MRO are correlated with macroscopic observables such as the kinetic fragility of the liquids and the elastic properties of the glasses. These findings allow to reach a deeper understanding of the nature of the MRO in network glass formers, insight that is crucial for establishing quantitative relations between their MRO and macroscopic properties.

In order to explore the structural mechanisms responsible for the plastic behavior andpolyamorphism in silica glasses v-SiO2, we have performed Molecular Dynamics (MD)simulations using different approaches. Recently, a Density Functional Tight-Binding(DFTB) (1) study has shown that the structural changes from low- to high-densityamorphous structures in v-SiO2 occur through a sequence of percolation transitions (2).These transitions also explain some of the mechanical properties of v-SiO2.To gain a deeper insight into the properties of the clusters and percolations networks, wehave performed classical MD simulations using a reliable pair potential (3). Due to its lowcomputational load, this method has allowed us to significantly increase the size of thesimulated system and have access to large length scales, from 2.5 to 12.0 nm. Similarpercolation transitions have been identified also for these new models. Generating severalorders of magnitude of cluster sizes allows the extraction of scaling laws associated witheach property of the clusters such as mass, correlation length, order parameter etc... Thescaling of such properties is correlated to the critical exponents or the fractal dimensionalityof the clusters. The collection of these quantities related to the transitions for the differentconnectivities SiO4-SiO4, SiO5-SiO5, SiO6-SiO6 and SiO6-stishovite, leads to thecharacterization of the percolation model, providing compelling evidence for amorphous-amorphous “phase” transitions in compressed silica glasses.

The fracture of oxide glasses is a subject of high complexity since many factors (e.g. length scale under investigation, strain rate, chemical composition, etc...) play an important role. Using large scale molecular dynamics simulations, we have investigated the composition dependence on the fracture behaviour of sodium silicate glasses on the microscopic scales. While silica glass presents a nearly perfect brittle fracture behaviour, we have found that the one of sodium rich glasses is accompanied by the nucleation of irregular voids as large as 3-4nm ahead of the crack front, indicating the presence of nanoductility for these glasses. We have also explored the spatial and temporal changes of various atomic-level properties and the correlations between them. It has been found that these properties are spatially very heterogeneous with disorder becoming more pronounced for alkali rich compositions. Close to the crack tip, a heating of several hundred degrees above the average temperature has been identified, which permits the structure to relax.

Despite the many experimental, theoretical, and numerical studies, there are still many unknowns about the microscopic understanding of the surface properties of silicate glasses. Nowadays, many applications rely on the control of surface composition, local structure, or morphology. Moreover, the disordered nature of the glass structure makes exploring and rationalizing some of these properties difficult on a microscopic level. MD simulations have been carried out to systematically examine the influence of alkali content and their chemical nature on the properties of alkali glass surfaces. We have shown how the production history, whether it's a melt-quench process or dynamic fracture, has a direct impact on both the atomistic-level and morphological properties

Nowdays there is an increasing need to control the properties of glasses and the ones of their surfaces, in particular. A considerable number of experimental and computational studies have been conducted in order to better describe the surface features of silicate glasses. Nevertheless there is still a lack of a deeper insight on how the surface topography and the structural environments depend on glass composition and/or production history. etc.Using molecular dynamics simulations, we have investigated the surface properties of alkali silicate glasses. We have studied two types of surfaces, a melt-formed surface and a fracture surface and the influence of the alkali modifier has been rationalized in terms of the interplay between multiple factors involving the size of the ions, bond strength, and charge balance on the surface. Specifically, we have also studied the effects of the glass composition and alkali modifier on the topographical properties of the two types of surfaces