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

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