Intermediate-range structure and conductivity of fast ion-conducting borate glasses

Abstract

The structures of the fast ion-conducting glasses LiCl–Li 2O–2B 2O 3, NaCl–Na 2O–2B 2O 3 and (AgI) 0.6–(Ag 2O–2B 2O 3) 0.4 have been studied by diffraction experiments and Reverse Monte Carlo (RMC) modelling. The results show that the glasses have different intermediate-range structures and also different local surroundings around their conducting cations. These differences are most likely caused by the different ionic-covalent characters of the lithium, sodium and silver ions and the different sizes and polarizabilities of the chlorine and iodine ions. The larger covalent character of the silver ions make them more active in the network formation and, therefore, they have larger influence on the intermediate-range structure than the more ionic lithium and sodium ions. The AgI doped glass has a lower average M–O coordination number, a more pronounced intermediate-range order and a larger accessible free volume for the dopant ions, which seems to be important for high ionic conductivity.