Molecular dynamics study of the structure and dynamic behavior at the surface of a silicate glass

Molecular dynamics calculations of a complex SiO2+B2O3+Al2O3+ZrO2+Na2O+CaO glass with surfaces revealed the formation of outer layers having different structural properties than the bulk glass. Alkali cations tend to migrate toward the surface, lowering the coordination number of the trivalent elements in the subsurface layer. The outer layer is also enriched in oxygen. Depolymerization leads to the formation of slightly larger rings on average. Atom mobility is enhanced in the lower density layers, as shown by the application of an external electric field and during simulated displacement cascades. Although these results are consistent with previously published findings, it is difficult to assess the actual scope of these surface effects (estimated by molecular dynamics to extend to a depth of about 10 A) because this approach does not take into account the effects of long-range diffusion or of the interface with the surrounding environment.