RESUMO
The structural properties of glassy diboron trioxide, g-B2O3, are investigated from ambient to high pressure conditions using two types of atomic force-field models that account for many-body effects. These models are parameterized by a dipole- and force-fitting procedure of reference datasets created via first-principles calculations on a series of configurations. The predictions of the models are tested against experimental data, where particular attention is paid to the structural transitions in g-B2O3 that involve changes to both the short- and medium-range order. The models outperform those previously devised, where improvement originates from the incorporation of two key physical ingredients, namely, (i) the polarizability of the oxide ion and (ii) the ability of an oxide ion to change both size and shape in response to its coordination environment. The results highlight the importance of many-body effects for accurately modeling this challenging system.
RESUMO
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 (BIV) and triangular BO3 (BIII). One of the salient features of these compounds is the change of the BIII/BIV 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 BIII/BIV with respect to the composition and the temperature.