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

TL;DR

This study investigates how the electrical conductivity and mechanical properties of AgI-AgPO3 glasses change with composition, revealing elastic phase transitions that influence ion mobility and conduction mechanisms.

Contribution

It introduces a self-organized ion hopping model that links mechanical constraints with ionic conduction, explaining compositional trends and phase transitions in glass electrolytes.

Findings

Identified two compositional thresholds near stress and rigidity transitions.

Observed step-like and exponential changes in conductivity related to elastic phases.

Concluded that ion mobility is primarily affected by network softening rather than carrier concentration.

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.