Nearly constant loss behavior in γ-RbAg4I5: microwave conductivity plateau identified

A pronounced nearly constant loss (NCL) contribution to the frequency-dependent conductivity has been detected in the low-temperature γ-phase of the archetypal crystalline fast ion conductor, rubidium silver iodide. At 113 K, the conductivity spectrum, σ(ν), of γ-RbAg4I5 has been measured in a wide frequency range extending up to more than 100 GHz. In contrast to the phases α and β, the spectrum cannot be interpreted as a superposition of a high-frequency vibrational part plus a low-frequency dispersive component that reflects the ionic hopping dynamics responsible for macroscopic ionic transport. Rather, an additional NCL component is identified featuring a roughly linear σ(ν)∝ ν behavior within about two decades of frequency. This component is found to merge into saturation at high frequencies and into a σ(ν)∝ ν2 behavior at low frequencies. It is well reproduced in terms of a model describing highly correlated, strictly localized displacive movements of interacting ions. The model is based on the concept of mismatch and relaxation (CMR).