Dynamics of mobile ions in glass - What do conductivity spectra tell us?

In glassy electrolytes, the diffusive motion of the mobile ions consists of thermally activated hops from site to site. Beyond this statement, little is known about the microscopic dynamics of ionic transport in glass, the pr oblems originating mainly from the lack of long-range order. An important s tep forward has been made recently by employing the technique of conductivi ty spectroscopy in a frequency range that covers about fourteen decades, ex tending up to the far infrared. This particular experimental tool acts as a "microscope in time" resolving hopping processes down to the sub-picosecon d time regime. The power of the method is exemplified for the case of a lit hium-ion conducting lithium bromide-lithium berate glass, which is represen tative in many respects. Among the results obtained is the frequent occurre nce of correlated back-and-forth hopping processes as well, as the finding that ions have preferences for optimally configured sites which play the ro le of stepping stones for translational diffusion.