A NEW MODEL OF IONIC TRANSPORT FOR SINGLE-ALKALI AND MIXED-ALKALI OXIDE GLASSES BASED ON ABINITIO MOLECULAR-ORBITAL CALCULATIONS

A new mechanism of ionic transport for single- and mixed-alkali silicate glasses is presented on the basis of ab initio molecular orbital calculations on a clusters modeling of the local structure of alkali silicate glasses. The calculations clearly show that an alkali ion site is energetically matched with the occupying alkali ion; the distortion energies caused by the transfer of an alkali, A, to a vacant site previously occupied by an alkali, B, are calculated to be almost-equal-to 67 to almost-equal-to 670 kJ/mol. Owing to the large distortion energies, it is proposed that alkali ions in mixed-alkali glasses move through the glasses by an interchange with nearby silicons. This type of alkali transport is called the interchange transport process'. It is characterized by random jumps and is most likely the origin of the mixed-alkali internal-friction peak. The mixed-alkali effect is well interpreted in terms of this type of alkali transport. That is, minor alkalis in mixed-alkali glasses do not migrate along the conduction pathways taken by major alkalis but make uncorrelated motions by the interchange transport process. Similarly, major alkalis migrate around minor alkalis by the interchange transport process. Further, it is shown that the interchange transport process plays an important role in the transport mechanism of modifier cations in low alkali ( < a few mol%) glasses in which conduction pathways are not established. The movement of alkali ions in alkali aluminosilicate and water-containing glasses is also discussed in terms of the mechanism presented.