  The progress in understanding the behavior of glassy mixed ionic conductors
within the concept of the defect model for the mixed mobile ion effect (V.
Belostotsky, J. Non-Cryst. Solids 353 (2007) 1078) is reported. It is shown
that in a mixed ionic conductor (e.g., mixed alkali glass) containing two or
more types of dissimilar mobile ions of unequal size sufficient local strain
arising from the size mismatch of a mobile ion entering a foreign site can not
be, in principle, absorbed by the surrounding network-forming matrix without
its damage. Primary site rearrangement occurs immediately, on the time scale
close to that of the ion migration process, through the formation of intrinsic
defects in the nearest glass network. Neither anelastic relaxation below glass
transition temperature, Tg, nor viscoelastic or viscous behavior at or above Tg
can be expected being observed in this case because the character of the stress
relaxation in a wide temperature range is dictated above all by the deformation
rates employed locally to the adjacent network-forming matrix. Since the ion
migration occurs on the picosecond time scale, the primary rearrangement of the
glass network adjacent to an ionic site occurs at rates orders of magnitude
higher than those of the critical minimum values, so the matrix demonstrates
brittle-elastic response to the arising strain even at temperatures well above
Tg, which explains, among other things, why mixed alkali effect is observable
in glass melts.
