It was recalled that the atomic environments of alkalies were affected by the presence of other alkalies; thus suggesting that they were juxtaposed in the glass structure. Mounting evidence from local structure spectroscopy (X-ray absorption fine structure, O magic angle spinning nuclear magnetic resonance) indicated that alkalies were coordinated mainly to non-bridging O atoms and were micro-segregated in silicate glasses. Reconfiguration of the O atoms was therefore expected to accompany the nearest-neighbor hopping of alkalies. Simple expressions were presented for the conformational energy that was associated with the latter hopping, and for the electrostatic energy contributions to the total microscopic energy barrier which confronted migrating alkali atoms. Cooperative effects which were reflected by the alternating-current conductivity were considered, and it was argued that these came into play at high concentrations of the alkali in the direct-current conductivity and self-diffusion behaviors. The macroscopic activation energy was related to the total microscopic energy barrier by dividing the latter by the Kohlrausch exponent for alternating-current conductivity. It was proposed that since, at alkali concentrations which were below the percolation threshold, the Kohlrausch exponent approached unity, the alkalies would become de-coupled and hopping to more distant sites would become more frequent. Here, the macroscopic activation energy would be approximately equal to the binding energy of the alkali. In the case of mixed alkali glasses, similar considerations were expected to apply to each of the alkalies. The majority alkali would exhibit mainly nearest-neighbor hopping (largely unaffected by the presence other alkalies) while the minority alkali, because it was largely surrounded by foreign alkalies, would be constrained to hop to more distant sites. Simple hopping expressions were used to predict the pre-exponential values for glasses in terms of the alkali concentration, the hopping distance and the hopping attempt frequency. All of these data could be obtained from the glass composition, and from X-ray absorption and far-infrared absorption results.

G.N.Greaves, K.L.Ngai: Journal of Non-Crystalline Solids, 1994, 172-174, 1378-88