A pronounced nearly constant loss (NCL) contribution to the frequency-dependent conductivity has been detected in the low-temperature gamma-phase of the archetypal crystalline fast ion conductor, rubidium silver iodide. At 113 K, the conductivity spectrum, sigma(v), of gamma-RbAg4I5 has been measured in a wide frequency range extending up to more than 100 GHz. In contrast to the phases alpha and beta, 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 sigma(v) proportional to v behavior within about two decades of frequency. This component is found to merge into saturation at high frequencies and into a sigma(v) proportional to v(2) 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). (C) 2004 Elsevier B.V. All rights reserved.