Ionic conduction in B₂S₃-Li₂S-LiI glasses

Abstract

The influence of the addition of the LiI "doping" salt to glasses with general composition (1-y)[(1-x)B₂S₃.xLi₂ S]yLiI (x=0, and 0.5×0.75) is studied, knowing that LiI does not affect the nature of the structural entities present in the base binary glasses. The glass transition temperature decreases when y increases. This effect is smaller for the largest values of x for which the structural entities are smaller and possess more non-bridging sulfurs. The density increases with y, which leads to a slight decrease in the Li⁺ concentration and to a rather constant Li-Li distance. Whatever the Li₂S amount, the conductivity activation energy goes through a weak but definite minimum upon addition of LiI and corresponding break is observed in the pre-exponential factor, which is not predicted by the Anderson-Stuart model. However, it is suggested that this model does not apply to the present glasses, which are rather highly modified. The conductivity data are interpreted by considering two different composition regimes based on the LiI concentration. Isolated I^- ions are present at low concentrations, which leads to a decrease in the Coulombic part of the activation energy through the replacement of smaller S^- ions by larger I^- ones. At higher concentrations, LiI aggregates start to form, which constitute high-barrier regions thus leading to an overall increase in the activation energy.