Low-Energy Ion Scattering spectroscopy of silicate glass surfaces

• The composition of the top atomic surface layer of silicate glasses was studied.
• The as-cast and the as-fractured glass surface compositions were different.
• As-cast surface depleted of the trisilicate glass modifier elements (Na, Cs, Ba)
• As-fractured surface enriched with Na and Cs, but depleted of Ba
• So-called “ideal”, as-fractured surface does not represent bulk glass stoichiometry.

Low-Energy Ion Scattering (LEIS) spectroscopy is a technique with a unique sensitivity to the elemental composition of the surface top atomic layers of a solid. LEIS measurements of simple binary silicate glasses (with Na2O, Cs2O or BaO) have shown differences in composition between the as-cast glass surface and the vacuum-fracture surface. The as-cast surface is usually depleted of modifier ions compared to the nominal (batch) bulk glass composition, while the fracture surface is typically enriched with the monovalent modifier species, but not the divalent barium. While the melt surface depletion can probably be attributed to evaporation from the hot melt surface, the origin of the strong accumulation on the fracture surfaces is less clear. It is certainly possible that the fracture may have been preferentially guided by regions already alkali-enriched within the bulk of the glass. But considering the extreme surface sensitivity of this method, a surface-monolayer localized enrichment with alkali due to the stress gradient created by the fracture or the electric field created by dangling bonds on the vacuum-fractured surface is also possible. The Si/O ratio shows usually little deviation from the nominal glass batch value, falling within the estimated errors.