Electronic and ionic conductivities in superionic Li${}_4$C${}_{60}$

Abstract

The $10$ GHz microwave conductivity, $\sigma (T)$ and high field, $222$ GHz electron spin resonance (HF-ESR) of Li${}_4$C${}_{60}$ fulleride is measured in a wide temperature range. We suggest that the majority of ESR active sites and at least some of the charge carriers for $\sigma (T)$ are electrons bound to a small concentration of surplus or vacancy ions in the polymer phase. Both $\sigma (T)$ and the ESR line shape depend on ionic motion. A change of the activation energy of $\sigma (T)$ at $125$ K coincides with the onset of the ionic DC conductivity. The ESR line shape is determined mainly by Li ionic motion within octahedral voids below $150$ K. At higher temperatures, fluctuations due to ionic diffusion change the environment of defects from axial to effectively isotropic on the ESR time scale. $\sigma (T)$ data up to $700$ K through the depolymerization transition confirm that the monomeric phase of Li${}_4$C${}_{60}$ is a metal.