  This paper presents a new defect model for the mixed mobile ion effect. The
essential physical concept involved is that simultaneous migration of two
unlike mobile ions in mixed ionic glass is accompanied by expansion or
contraction of the guest-occupied sites with distortion of surrounding glass
matrix; in many cases, an intensity of the local stresses in glass matrix
surrounding ionic sites occupied by foreign ions is much greater than, or at
least comparable to the glass network binding energy. Hence, when the stress
exceeds the breaking threshold, relaxation occurs almost immediately via the
rupture of the bonds in the nearest glass matrix with generation of pairs of
intrinsic structural defects. The specificity of the mechanism of defect
generation leads to the clustering of negatively charged defects, so that
rearranged sites act as high energy anion traps in glass matrix. This results
in the immobilization of almost all minority mobile species and part of
majority mobile species, so mixed mobile ion glass behaves as single mobile ion
glass of much lower concentration of charge carriers. Generation of defects
leads also to the depolymerization of glass network, which in turn results in
the reduction of the glass viscosity and Tg as well as in the compaction of
glass structure (thermometer effect). The magnitude of the mixed mobile ion
effect is defined by the size mismatch of unlike mobile ions, their total and
relative concentrations, the binding energy of the glass-forming network, and
temperature. Although the proposed model is based upon the exploration of
alkali silicate glass-forming system, the approach developed here can be easily
adopted to other mixed ionic systems such as crystalline and even liquid ionic
conductors.
