Molecular-dynamics simulations of (NaO2)x(SiO2)1-x glasses: Relation between distribution and diffusive behavior of Na atoms

Abstract

Molecular-dynamics simulations of sodium silicate glasses in a range of alkali concentration going from 1.8% to 33.33% molar of Na2O are reported. Our simulations show that there is a tight relationship between Na atoms and nonbridging oxygens which are mainly located in the first Na coordination shell. In the whole range of composition, Na and nonbridging oxygen atoms appear to segregate giving rise to a heterogeneous distribution. For the higher alkali concentrations, formation of microchannels is observed. The activation energies for alkali diffusion have been computed and found in agreement with the experiment. The mechanism for the diffusion has also been investigated and found to occur through the microchannels, if present, or across the network for low compositions, but the Na motions always appear to be assisted by nonbridging oxygen atoms.