Low- and high-frequency conductivities of RbAg4I5 and Ag β″-alumina at low temperatures

Abstract The specific ionic conductivities, σ, of polycrystalline RbAg 4 I 5 and Ag β″-alumina superionic conductors were determined by two- and four-probe impedance measurements at low frequencies (10 −3 Hz≤ƒ≤10 2 Hz) and waveguide measurements at high frequencies (18×10 9 Hz≤ƒ≤26.5×10 9 Hz≡:k-band) in the temperature range 15K≤ T ≤298K. Above a transition temperature T t =(90±20)K, the conductivity data represent clearly a silver ion transport process with thermal activation. The activation energies of the α, β, and γ phases of RbAg 4 I 5 were determined for the low- and high-frequency ranges. Below T t a nearly temperature-independent conductivity was observed in accordance with previous results. Around T t a long-time relaxation effect is observed in low-frequency EIS measurements, changing α and the dielectric constant e. The low-temperature conductivity data can be explained neither in terms of the jump-relaxation nor in terms of the two-level tunneling system (TLS) model. A different mechanism which takes into account the influence of inhomogeneities on the ionic conductivity is proposed. This mechanism is obviously valid in a restricted temperature range located between the ion TLS-tunneling and the jump-relaxation regimes.