THE STRUCTURE OF SUPERIONIC GLASSES FROM NEUTRON-DIFFRACTION AND REVERSE MONTE-CARLO SIMULATIONS

We have performed neutron diffraction experiments on metal-halide doped superionic glasses and combined them with Reverse Monte Carlo (RMC) simulations to investigate the microscopic structure and its relation to the ionic conductivity. The experiments reveal a substantial change and the building up of a new type of intermediate range order as silver halide dopant salts are introduced into the glass network, whereas the short range order of the network is unaffected. It is observed as an extra diffraction peak at anomalously low Q-values (0.7-0.8 angstrom-1) for the doped glasses. The RMC simulations show that the anomalous peak is due to local density fluctuations in the host glass network caused by the requirement to maintain connectivity (i.e. bonding) while decreasing the average density. The simulations indicate separated AgI and host glass networks in the glass. Some features of the radial distribution function of the doped glass may be interpreted as indicating limiting microscopic fractal aspects of the expanded host network. The results are discussed in relation to structure-conductivity models suggested for superionic glasses.