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.