Cation microsegregation and ionic mobility in mixed alkali glasses

MUCH is known about short-range (< 5 angstrom) structural order in oxide glasses from experimental probes of local structure such as X-ray absorption fine structure (XAFS) 1, but over the medium range (5-20 angstrom) their structures are poorly understood. Computer simulations based on measured parameters for local atomic environments, however, can provide structural models on the nanometre scale, which enable dynamic properties such as ionic transport to be considered. Here we describe a molecular dynamics simulation of the effects of mixed alkali cations on the structure of binary silicate glasses. It is well known that the ionic conductivity of alkali glasses falls markedly when more than one alkali is present 2. We demonstrate that the alkalis segregate from the silicate network over distances of a few angstroms. Although ionic mobility is expected to be higher in these microsegregated regions than in the surrounding silicate network, we suggest that stochastic mixing of alkalis nevertheless impedes the hopping process of a given alkali ion. This is manifest as an increase in the average activation energy for hopping and results in a lowering of the total ionic conductivity.