Fast-ion conduction and flexibility and rigidity of solid electrolyte glasses

Electrical conductivity of dry, slow cooled (AgPO₃)_1-x(AgI)ₓ glasses is examined as a function of temperature, frequency, and glass composition. From these data compositional trends in activation energy for conductivity EA(x), Coulomb energy Ec(x) for Ag⁺ ion creation, Kohlrausch stretched exponent β(x), low-frequency [εₛ(x)] and high-frequency [ε_∞(x)] permittivity are deduced. All parameters except Ec(x) display two compositional thresholds, one near the stress transition, x=xc(1)=9%, and the other near the rigidity transition, x=xc(2)=38% of the alloyed glass network. These elastic phase transitions were identified in modulated differential scanning calorimetry, IR reflectance, and Raman-scattering experiments earlier. A self-organized ion-hopping model of a parent electrolyte system is developed that self-consistently incorporates mechanical constraints due to chemical bonding with carrier concentrations and mobility. The model predicts the observed compositional variation in σ(x), including the observation of a steplike jump when glasses enter the intermediate phase (IP) at x>xc(1), and an exponential increase when glasses become flexible at x>xc(2). Since Ec is found to be small compared to network strain energy (Eₛ), we conclude that free carrier concentrations are close to nominal AgI concentrations, and that fast-ion conduction is driven largely by changes in carrier mobility induced by an elastic softening of network structure. Variation in the stretched exponent β(x) is square-well like with walls localized near xc(1) and xc(2) that essentially coincide with those of the IP (xc(1)<x<xc(2)), and suggest filamentary (quasi-one-dimensional) conduction in the IP, and conduction with a dimensionality greater than 1 outside the IP.