Structure and vibrational properties of sodium silicate glass surfaces
Résumé
Using molecular dynamics simulations we investigate the dependence of the structural
and vibrational properties of the surfaces of sodo-silicate glasses on the sodium content as well as the nature of the surface. Two types of glass surfaces are considered:
A melt-formed surface (MS) in which a liquid with a free surface has been cooled
down into the glass phase and a fracture surface (FS) obtained by tensile loading
of a glass sample. We find that the MS is more abundant in Na and non-bridging
oxygen atoms than the FS and the bulk glass, whereas the FS has higher concentration of structural defects such as two-membered rings and under-coordinated Si than
the MS. We associate these structural differences to the production histories of the
glasses and the mobility of the Na ions. It is also found that for Na-poor systems the
fluctuations in composition and local atomic charge density decay with a power-law
as a function of distance from the surface while Na-rich systems show an exponential
decay with a typical decay length of ≈ 2.3 Å. The vibrational density of states shows
that the presence of the surfaces leads to a decrease of the characteristic frequencies
in the system. The two-membered rings give rise to a pronounce band at ≈ 880 cm−1
which is in good agreement experimental observations.
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