Structure of AgI-Ag2O-2B(2)O(3) glasses: A neutron and x-ray-diffraction investigation

The structures of fast ion conducting glasses in the system (AgI)(x)-(Ag2O-2B(2)O(3))(1-x) have been investigated using x-ray- and neutron-diffraction techniques. The contrasting scattering cross sections of the two techniques for the different atomic species, where the x rays reveal predominantly the Ag and I structural correlations, whereas mainly the boron-oxygen structure is observed with the neutrons, allow for a relatively detailed structural determination of these complex materials. The neutron data show that the short-range order of the boron-oxygen network, which determines the network connectivity, is unaffected by the AgI doping. The silver and iodine ions enter voids in the boron-oxygen network structure. The experimental data also show the existence of considerable intermediate-range ordering which increases with increasing AgI doping. This is manifested in a diffraction peak at low Q (around 0.8 Angstrom(-1)), which grows with increasing AgI content and which is particularly strong in the neutron data. The contrasting results of the x-ray- and neutron-diffraction experiments show unambiguously that this ordering is mainly due to correlations within the baron-oxygen network. The experimental data have been used for structural modeling using the reverse Monte Carlo method. Structural models have been produced which are simultaneously in accordance with bath the x-ray and the neutron data, as well as with the experimental density. The simulations show that the intermediate-range ordering is due to interchain ordering between neighboring chain segments. Analysis of the models shows that most silver ions are coordinated to both iodine and oxygen ions of negatively charged BOB units in the network. This observation, as well as the evidence that the intermediate-range ordering observed is due to the boron-oxygen network, excludes the presence of any significant amount of AgI clusters, which have been suggested in the literature to be responsible for the high ionic conductivity of AgI-doped oxyglasses. instead the results support the idea that the silver and iodine ions expand the network which open doorways and creates new pathways in the structure suitable for ion conduction.