Role of Na in determining the properties of sodic glasses

Abstract

Molecular Dynamics (MD) simulation of Na 4 SiO 4 glass yields Na-O bond distances, and O-Na-O angles remarkably similar to those in sodic crystals, regardless of their compositions. Na polyhedra in both phases are stabilized by a common, formative mechanism, collapse of SiO 4 tetrahedra around Na, driven by Na-O electrostatic attraction (electrostriction). Na-O bond distance distribution in the simulated glass displays two maxima at ∼2.38Å and ∼4.8Å. The first Na-O coordination sphere is strongly skewed with a cut-off distance of ∼3.25Å. At this extreme, Na-O bonds are highly ionic. These results provide an energetics explanation for Na migration in glasses. Five-fold coordinated Na, combined with synchronicity of Na-O and Si-O symmetric stretches, promote Na migration. Density functional theoretical (DFT) calculations on Na-Si-O molecular species reveal that Na bonded to SiO 4 tetrahedral edges is more stable than apex- or face-bonded Na. Also, Si-O-Si linkages are stabilized by having Na bonded to bridging oxygen.