Mechanical relaxation anomalies in mixed alkali oxides

Mechanical relaxation (MR) processes were investigated in single and mixed alkali (MA) metaphosphate glasses using a dynamic mechanical analyzer (DMA) over a range of frequencies, 0.1-50 Hz, and temperatures, 24-250 degrees C. The mechanical loss modulus, M ", of each mixed, sodium and lithium, alkali glass exhibited two characteristic maxima, a large maximum just below T-g, and a well developed, yet considerably diminished in amplitude, maximum at a much lower temperature. The single alkali analogs, on the other hand, exhibited only a single maximum and this maximum appeared in the same location as the lower temperature peak observed in the MA glasses. The location of these maxima are identified with dynamic processes within the glass which occur with average frequencies, nu(mu)(Na, Li), for the high T maximum in the mixed glasses, and nu(mu) (Li) and nu(mu) (Na) for the lithium and sodium glasses, 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 largest activation energy. In addition nu(mu)(Na, Li) << nu(mu)(Na) < nu(mu)(Li) for T < T-g. Two other important observations were made, the high temperature maximum in M " reached its largest amplitude when the mole fractions of Na2O and Li2O were comparable and nu(mu) (Na, Li) exhibited a maximum in the same composition range. Our observations are discussed in light of a local site-memory relaxation model based on the notion that below T,, cation hopping dynamics are intimately coupled with local glass network relaxations. (C) 1998 Elsevier Science B.V. All rights reserved.