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

The $10$ GHz microwave conductivity, $σ(T)$ 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 $σ(T)$ are electrons bound to a small concentration of surplus or vacancy ions in the polymer phase. Both $σ(T)$ and the ESR line shape depend on ionic motion. A change of the activation energy of $σ(T)$ 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. $σ(T)$ data up to $700$ K through the depolymerization transition confirm that the monomeric phase of Li$_4$C$_{60}$ is a metal.

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