FRACTAL ASPECTS OF SUPERIONIC GLASSES FROM REVERSE MONTE-CARLO SIMULATIONS

Comparative neutron diffraction experiments on AgPO3 and AgI-AgPO3 glasses reveal large changes in the intermediate-range ordering as the dopant salt is introduced, whereas the short-range ordering of the host matrix is unchanged by the salt. The observations indicate a microscopically unaffected host network and space-filling microclusters of the dopant salt. It has been suggested that such AgI microclusters form diffusion pathways for the Ag ions and therefore are important for the conduction mechanism. In order to understand better the detailed structure of these complicated superionic glasses we have taken advantage of the recently developed reverse Monte Carlo simulation method which uses the experimental structure factor directly to produce a structural model. The models show that the PO network, when expanded by AgI doping, keeps its connectivity but contains large holes which are filled by the salt. It is the low-density PO network that ives rise to the sharp diffraction peak at anomalously low Q values (about 0.7 angstrom-1). Some features of the P-P partial radial distribution of the expanded network, which are not present in the undoped salt, may be interpreted as indicating some fractal aspect of the structure with an upper characteristic length of about 15-20 angstrom.