Defect model for the mixed mobile ion effect revisited: an importance of deformation rates

TL;DR

This paper revisits the defect model for the mixed mobile ion effect in glassy conductors, emphasizing the critical role of deformation rates in local strain and defect formation during ion migration.

Contribution

It demonstrates that local strain from size mismatch causes immediate defect formation, with deformation rates dictating stress relaxation behavior in mixed ionic glasses.

Findings

Local strain leads to defect formation during ion migration.

Deformation rates determine the stress relaxation mechanism.

Mixed alkali effect persists in glass melts due to rapid defect formation.

Abstract

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.