Electronic and ionic conductivities in superionic Li$_4$C$_{60}$
  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. 
D. Quintavalle, B. G. M\'arkus, A. J\'anossy, F. Simon, G. Klupp, M.   A. Gy\H{o}ri, K. Kamar\'as, G. Magnani, D. Pontiroli,...
cond-mat.str-el