Free volume and dissociation effects in fast ion conducting glasses

We have investigated the structural changes that occur on CsI and CsI-AgI d oping in AgPO3 using neutron diffraction and Raman scattering experiments. To obtain further insights into the microscopic structure of the glasses we have applied the reverse Monte Carlo (RMC) method to produce structural mo dels that are in quantitative agreement with the neutron diffraction data, as well as with the experimental density and applied physical constraints. The aim was to examine the structural role of the immobile (compared to Ag) Ca+ and I- ions for the increased ionic conductivity upon CsI doping. It is clear from the RMC and Raman results that the Cs+ ions coordinate oxygen s more than Ag+, and in this way Cs+ contributes to a partial dissociation of the Ag+ ions from the non-bridging oxygens. The iodine ions have a simil ar role by forming largely covalent bonds to silver ions, leading to a furt her decrease of the Ag-O coordination. The introduction of the salt ions ex pands the host glass network. We suggest that both the electrostatic bindin g energy, E-b, and the elastic strain energy, E-s, associated with the ioni c motion decrease with increasing CsI content due to the partial dissociati on of the silver ions from the non-bridging oxygens and the expansion of in terstices which are larger migration pathways for the silver ions. (C) 2000 Elsevier Science B.V. All rights reserved.