Structural Analysis of Molten Na2O-NaF-SiO2 System by Raman Spectroscopy and Molecular Dynamics Simulation

To determine the effect of F ions on the structure of the molten alkali silicate systems, quenched Na2O-SiO2-NaF systems were investigated by Raman spectroscopy and molecular dynamics simulation. The systematic increase of 1100 cm−1 band intensity in the Raman spectra of the silicate melts accompanying the replacement of O by F provides the evidence for concomitant polymerization of melts. From the molecular dynamics simulation, it was confirmed that most of substituted F was mainly coordinated to Na+ ions but not Si4+ ions at least up to 12.5 mol% of F ion content. A small amount of F was found to be coordinated to Si as a non-bridging ion from the molecular dynamics simulation, although there was no recognizable evidence from Raman spectroscopy. These results were consistent with the mechanism in which F associated with otherwise network-modifying Na rather than with network-forming Si. Since F was associated to Na+ ions, the replace of O ion by two F ions promote the polymerization of silicate melts.