Structural study of Na₂O-B₂O₃-SiO₂ glasses from molecular simulations using a polarizable force field

Sodium borosilicate glasses Na₂O-B₂O₃-SiO₂ (NBS) are complex systems from a structural point of view. Three main building units are present: tetrahedral SiO₄ and BO₄ (B^IV) and triangular BO₃ (B^III). One of the salient features of these compounds is the change of the B^III/B^IV ratio with the alkali concentration, which is very difficult to capture in force fields-based molecular dynamics simulations. In this work, we develop a polarizable force field that is able to reproduce the boron coordination and more generally the structure of several NBS systems in the glass and in the melt. The parameters of the potential are fitted from density functional theory calculations only, in contrast with the existing empirical potentials for NBS systems. This ensures a strong improvement on the transferability of the parameters from one composition to another. Using this new force field, the structure of NBS systems is validated against neutron diffraction and nuclear magnetic resonance experiments. A special focus is given to the distribution of B^III/B^IV with respect to the composition and the temperature.