We present the study of the behaviour of amorphous silica under compression within the framework of computer simulations. In contrast to previous similar numerical works, our investigation is an exploratory study using finite-range pair-potential in order to get insight into the structural modications on both local and intermediate length scales when the pressure/density is increased. We have hence identified the characteristic bond distances and angles of Si-coordination polyhedra (SiO5 and SiO6) formed under high pressure. We wiil equally consider the plastic deformation upon applied stress and discuss the nature of this deformation upon loading and unloading.

We use large-scale molecular dynamics simulations to study the kinetics of the liquid-gas phase separation if the temperature is lowered across the glass transition of the dense phase. We observe a gradual change from phase separated systems at high temperatures to nonequilibrium, gel-like structures that evolve very slowly at low temperatures. The microscopic mechanisms responsible for the coarsening strongly depend on temperature, and change from diffusive motion at high temperature to a strongly intermittent, heterogeneous and thermally activated dynamics at low temperature, leading to logarithmically slow growth of the typical domain size.

We have carried out first-principles molecular dynamics simulations of a sodium borosilicate (NBS) glass within the framework of DFT. The composition is 6SiO2-3Na2O-B2O3 and the density was fixed to 2.51g.cm-3. The system size is 320 atoms (i.e. 60 silicon, 180 oxygen, 60 sodium and 20 boron atoms) which is rather demanding for ab-initio simulations and has been chosen to obtain a good statistics, in particular for quantities associated to the lowest concentrated element (boron). The simulations have been started at 4500K in the canonical ensemble (NVT) and followed by a simulation in the microcanonical ensemble (NVE). Subsequently, the system is progressively cooled to 300K within 60ps. In order to characterized the structure of the system we have calculated the pair correlation functions, the angular distributions, the Qn distributions, the non bridging oxygen's concentration as well as the partial structure factors. Moreover, we have studied the environment of 3- and 4-fold (BIII and BIV) coordinated boron atoms. Furthermore, the vibrational density of states has been studied in details. Indeed, both BIII and BIV atoms have specific frequencies, and the partial vibrational density of the 3-fold coordinated B atoms has been found to be a weighted sum of two specific contributions: 3-fold symmetric coordinated B (BIIIs) atoms and asymmetric coordinated B (BIIIa) atoms. Identically, the partial vibrational density of the 4-fold coordinated B atoms has been found to depend on the nature of their second neighbor namely so-called BIV(4Si-0B), BIV(3Si-1B), BIV(2Si-2B), BIV(1Si-3B), and BIV(0Si-4B) respectively with 4 Si and 0 B atoms as second neighbor and so on. The infrared spectra of our glassy models is also presented.

First-principles techniques have been employed to study the reactivity of water into a calcium aluminosilicate glass. In addition to the well known hydrolysis reactions Si-O-Si+h2O->Si-OH + Si-OH and Si-O-Al+h2O-> Si-OH+Al-OH, a peculiar mechanism is found, leading to the formation of an AlO3–H2O entity and the breaking of Al–O–Si bond. In the glass bulk,most of the hydrolysis reactions are endothermic. Only a few regular sites are found reactive (i.e. in association with an exothermic reaction), and in that case, the hydrolysis reaction leads to a decrease of the local disorder in the amorphous vitreous network. Afterwards, we suggest that ionic charge compensators transform into network modifiers when hydrolysis occurs, according to a global process firstly suggested by Burnham in 1975. Our theoretical computations provide a more general model of the first hydrolysis steps that could help to understand experimental data and water speciation in glasses.