Mixed-alkali effect in sodium-potassium glasses irradiated with electrons

Mixed-alkali effect (MAE) was studied by irradiating (15 - x)Na(2)O center dot xK(2)O center dot 85SiO(2) glasses with electrons and observing the following glass response expressed through volume changes measured with AFM. Glasses were irradiated by 50 keV electrons with doses up to 191 kC/m(2). Low irradiation doses showed the volume compaction, extent of which was linearly dependent on potassium content. It was shown the compaction can be explained as the superposition of the relaxation around alkali ions and the silica network relaxation. At high doses most of glasses show volume expansions. High irradiation doses cause alkali depletion from the surface glass layer and reveal a typical MAE response in volume changes. it is explained in terms of negative correlation among the element specific migration paths. An alternative view of MAE is suggested: the coupling among moving ions is replaced by the correlation among migration paths that are predetermined in the structure of mixed glasses. (C) 2009 Elsevier B.V. All rights reserved.