It was recalled that fast ionic conductivity could be obtained by doping modified oxide glasses with a metal halide salt. By using conductivity and density data on various host glasses, mixed with various metal halide salts, it was found that a cubic scaling relationship existed between the conductivity enhancement and the expansion of the network-forming units that was induced by doping. It was concluded that the glass network expansion, which was related to the available free volume, was a key parameter which governed the increase in ionic conductivity of this type of fast ion conducting glass

J.Swenson, L.Börjesson: Physical Review Letters, 1996, 77[17], 3569-72