An investigation of the structure of oxide glasses using EXAFS
The atomic species sensitivity of EXAFS lends itself to the investigation of the local atomic structure of certain cations within multi-component disordered systems. To this end, X-ray absorption measurements were performed on two systems of glasses, containing Zr02 and Ti02 as major constituents. Both of these oxides are known to confer technologically important properties, however, their exact structural role is far from well understood. For the case of Zr02, the R20-CaO-Zr02-Si02 (R = Li, Na, K) system was chosen and structural changes were monitored as a function of alkali type and content and by the method of preparation. For annealed specimens, the results show a considerably ordered arrangement of, on average, six oxygens at 2.09 ± 0.05 A, indicating that the Zr site can be classified as network forming. The comparison of annealed and quenched specimens show that structural changes are observable as a result of heat treatment below Tg. Previous investigations of Ti02-containing glasses have shown that the coordination of Ti4+ ions varies as a function of composition. Such effects should be observable by the EXAFS technique and a series of Na20-Ti02-Si02 glasses were measured. Furthermore, the Ti K-edge is characterized by transitions to bound states of the excited atom (XANES) which reflect the symmetry of the surrounding atoms. Results using both of these phenomena support the overall trend of increasing low coordination states (<6) with increaSing Ti02 concentration, although discrepancies with other works are observed at low Ti02 content, where tetrahedral coordination is still observed, which may be due to the method of glass preparation. At the compositional ratio Si02:Ti02 ~ 1-2 a maximum in this coordination, thought to be due to isostructural substitution of Si4+ ions, is obseryed as a function of Na20 content. Further addition of Ti02 results in phase separation.