Free volume and dissociation effects in fast ion conducting glasses

We have investigated the structural changes that occur on CsI and CsI–AgI doping 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 models 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+) Cs+ 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 oxygens 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 similar role by forming largely covalent bonds to silver ions, leading to a further decrease of the Ag–O coordination. The introduction of the salt ions expands the host glass network. We suggest that both the electrostatic binding energy, Eb, and the elastic strain energy, Es, associated with the ionic motion decrease with increasing CsI content due to the partial dissociation of the silver ions from the non-bridging oxygens and the expansion of interstices which are larger migration pathways for the silver ions.