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

Abstract

Electrical conductivity of dry, slow cooled (AgPO$_3$)$_{1-x}$(AgI)$_x$ glasses is examined as a function of temperature, frequency and glass composition. From these data compositional trends in activation energy for conductivity E$_A$(x), Coulomb energy E$_c$(x) for Ag$^+$ ion creation, Kohlrausch stretched exponent $\beta$(x), low frequency ($\varepsilon_s$(x)) and high-frequency ($\varepsilon_\infty$(x)) permittivity are deduced. All parameters except E$_c$(x) display two compositional thresholds, one near the stress transition, x = x$_c$(1)= 9%, and the other near the rigidity transition, x = x$_c$(2)= 38% of the alloyed glass network. These elastic phase transitions were identified in modulated- DSC, IR reflectance and Raman scattering experiments earlier. A self-organized ion hopping model (SIHM) 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 of $\sigma$(x), including the observation of a step-like jump when glasses enter the Intermediate Phase at x$>$x$_c$(1), and an exponential increase when glasses become flexible at x$>$x$_c$(2). Since E$_c$ is found to be small compared to network strain energy (E$_s$), 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.