Correlation between free volume and ionic conductivity in fast ion conducting glasses

It is well known that fast ionic conductivity can be obtained by doping modified oxide glasses with a metal-halide salt. The role of the dopant salt, apart from providing additional charge carriers, for the ionic conductivity has been a much debated issue. Using conductivity and density data for different host glasses mixed with various metal-halide salts, we find a remarkable common cubic scaling relation between the conductivity enhancement and the expansion of the network forming units induced by salt. doping. This suggests that the glass network expansion, which is related to the available free volume, is a key parameter determining the increase of the high ionic conductivity in this type of fast ion conducting glasses.