The ionic conductivity in glass ceramics depends not only on the partial conductivities of the different phases and their volume fractions butto large extendon the structure of the composite material. Moreover, conductivity can be drastically increased by enhancement of ionic mobility along grain boundaries. The conductivity of glass ceramics is discussed in terms of upper and lower bounds depending on the degree of information about the structure. An effective medium approach is used to describe the dc conductivity for some characteristic types of structure. This approach is extended to the frequency dependence of conductivity. The model is applied to experimental data from glass ceramics of the system Si0,-PbO-PbF,. In this system different structures have been realized leading to an increase or decrease of conductivity by up to three orders of magnitude compared to the starting glass.

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Current models attribute the power law frequency dependence of ionic conductivity in oxide glasses alternatively to structural disorder or to interactions between moving ions. To distinguish between the influence of disorder and of interaction the frequency dependence of conductivity was investigated for three different series of glasses that have been selected according to special considerations on network structure and bonding of alkali ions. It was shown that the frequency response is independent of alkali concentration and structure. This result is discussed in terms of Coulomb interactions between mobile charged point defects and favours the jump relaxation model of Funke.

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