MECHANICAL RELAXATION ANOMALIES IN MIXED-ALKALI OXIDES

Mechanical relaxation (MR) processes were investigated in single and m ixed alkali (MA) metaphosphate glasses using a dynamic mechanical anal yzer (DMA) over a range of frequencies, 0.1-50 Hz, and temperatures, 2 4-250 degrees C. The mechanical loss modulus, M '', of each mixed, sod ium and lithium, alkali glass exhibited two characteristic maxima, a l arge maximum just below T-g, and a well developed, yet considerably di minished in amplitude, maximum at a much lower temperature. The single alkali analogs, on the other hand, exhibited only a single maximum an d this maximum appeared in the same location as the lower temperature peak observed in the MA glasses. The location of these maxima are iden tified with dynamic processes within the glass which occur with averag e frequencies, nu(mu)(Na, Li), for the high T maximum in the mixed gla sses, and nu(mu) (Li) and nu(mu) (Na) for the lithium and sodium glass es, respectively. These frequencies nu(mu)(Na, Li), nu(mu) (Li) and nu (mu) (Na), varied exponentially with 1/T; nu(mu) (Na, Li) had the larg est activation energy. In addition nu(mu)(Na, Li) << nu(mu)(Na) < nu(m u)(Li) for T < T-g. Two other important observations were made, the hi gh temperature maximum in M '' reached its largest amplitude when the mole fractions of Na2O and Li2O were comparable and nu(mu) (Na, Li) ex hibited a maximum in the same composition range. Our observations are discussed in light of a local site-memory relaxation model based on th e notion that below T,, cation hopping dynamics are intimately coupled with local glass network relaxations. (C) 1998 Elsevier Science B.V. All rights reserved.