Anomalous ionic conduction in AgBrAgI mixed crystals and multiphase systems

Ionic conductivity, σ, of the AgBrAgI system has been studied as a function of composition and temperature. The maximum conductivity of $\text{3 × 10}{}^{-4}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ at 25°C is obtained for a AgI-20 mole% AgBr two-phase mixture which is $\text{\$}\text{?}$3 orders of magnitude larger than that predicted by the classical theories of Lord Rayleigh and Maxwell. On the other hand, the substitution of so-called homovalent ions, e.g. Br^? in AgI and I^? in AgBr one phase solid solutions leads to anomalously large increase in the ionic conductivity that cannot be explained in terms of the charge compensation (doping) mechanism, and is attributed to purely elastic displacements (lattice distortion) due to the very “wrong” size of the substituted ions. A quadratic dependence of conductivity on the concentration of substituents is substantiated. An important consequence of the latter anomaly is that AgBr + 30 mole% AgI exhibits σ $\text{\$}\text{?}$7 Ω^?1 cm^?1 at 380°C which is $\text{\$}\text{?}$170% higher than that of α-AgI, the best known superionic conductor, at its melting point (557°C).