Abstract
Measurements of the electrical conductivity and tracer diffusion coefficient have been made in a series of XNa 2O : YAl 2O 3 : (1− X− Y) GeO 2 glasses where X ≈ 0.01, 0.05, 0.10, 0.15, 0.19 and 0.29 and Y/ X ≈ 0, 0.33 and 1.0. The dc conductivities of the sodium germanate glasses show a minimum in sodium ion mobility at X ∼ 0.09, that is a result of a maximum in the activation enthalpy for ion migration. The maximum in the activation enthalpy appears to be directly related to the packing density of the glass network forming ions. Measurements of the activation enthalpy for the sodium aluminogermanate glasses confirm this relationship between enthalpy and the packing density of the network forming ions. Examination of the pre-exponential factors for the present glasses (with or without alumina) show a strong dependence on sodium content. This dependence appears to be a direct result of the changes in the physical correlation factor, ƒ I, that ensue when the spacing between the sodium ions is changed. At low X the sodium ions are far apart and ƒ I is close to unity but as X increases and sodium ion separation decreases, ordering sets in and ƒ I is reduced.
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