First principles study of a sodium borosilicate glass-former I: The liquid state

Abstract

We use ab initio simulations to study the static and dynamic properties of a sodium borosilicate liquid with composition 3Na_2O-B_2O_3-6SiO_2, i.e. a system that is the basis of many glass-forming materials. In particular we focus on the question how boron is embedded into the local structure of the silicate network liquid. From the partial structure factors we conclude that there is a weak nanoscale phase separation between silicon and boron and that the sodium atoms form channel-like structures as they have been found in previous studies of sodo-silicate glass-formers. Our results for the X-ray and neutron structure factor show that this feature is basically unnoticeable in the former but should be visible in the latter as a small peak at small wave-vectors. At high temperatures we find a high concentration of three-fold coordinated boron atoms which decreases rapidly with decreasing T, whereas the number of four-fold coordinated boron atoms increases. Therefore we conclude that at the experimental glass transition temperature most boron atoms will be four-fold coordinated. We show that the transformation of [3]B into [4]B with decreasing T is not just related to the diminution of non-bridging oxygen atoms as claimed in previous studies, but to a restructuration of the silicate matrix. The diffusion constants of the various elements show an Arrhenius behavior and we find that the one for boron has the same value as the one of oxygen and is significantly larger than the one of silicon. This shows that these two network formers have rather different dynamical properties, a result that is also confirmed from the time dependence of the van Hove functions. Finally we show that the coherent intermediate scattering function for the sodium atoms is very different from the incoherent one and that it tracks the one of the matrix atoms.