The concept of matrix-mediated coupling: a new interpretation of mixed-cation effects in glass

The concept of matrix-mediated coupling assumes that coupling between movements of unlike cations in glass occurs in order to dissipate mechanical stresses created by small cations entering sites which are too large (e.g. Li+ ions entering empty sodium, (Na) over bar, sites) and large cations entering sites which are too small (e.g. Na+ ions entering empty lithium, (Li) over bar, sites). A central feature of the mixed-alkali effect is therefore the intervention of structural interactions that minimize these stresses; these processes are essentially isochoric in nature. This concept helps in the understanding of several important phenomena in addition to the high pressure effects discussed previously, including the growth/suppression effects observed in mechanical loss spectra; the strong effect of foreign cations on the conductivity in the dilute foreign alkali region; the role of divalent cations in glass and the 'anomalous' behaviour of ion-exchanged glasses.