Reconciling ionic-transport properties with atomic structure in oxide glasses

Abstract

Structural evidence for the microsegregation of alkalis in oxide glasses is reviewed and the implications for ionic transport, viz., nearest-neighbor hopping, cooperative and correlation effects, are considered. Distinctions are drawn between the hopping of alkalis in silicate glasses, where changes in the configurations of neighboring bridging and nonbridging oxygens are expected, and alkali hopping in fully compensated aluminosilicate glasses where nonbridging oxygens are absent and conformational changes in the network are minimized. A simple expression is introduced for the microscopic energy barrier facing a migrating alkali ${\mathit{E}}_{\mathit{a}}$, and this is corrected to account for the cooperative effects of the other ions involved by dividing by the Kohlrausch exponent β, which defines the conductivity relaxation function exp[-(t/${\mathrm{\tau }}^{\mathrm{*}}$)β]. Structural parameters determined by x-ray and NMR spectroscopy enable us to calculate ${\mathit{E}}_{\mathit{a}}$. Conductivity relaxation experiments give a measure of β. The macroscopic diffusion enthalpy that is measured, W, is given by the ratio ${\mathit{E}}_{\mathit{a}}$/β. Thus we are able to show how the local structure of an alkali in a silicate glass can be used to predict the measured diffusion enthalpy. The smaller values of W reported for aluminosilicate glasses are rationalized structurally in terms of the removal of nonbridging oxygens from the modified network. In considering silicate glasses containing small concentrations x of alkali, as this necessarily leads to reductions in alkali microsegregation, decreased cooperative effects and increased hopping distances are expected.