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

TL;DR

This study investigates the electronic and ionic conductivities of Li$_4$C$_{60}$ fulleride using microwave conductivity and ESR, revealing the interplay of ionic motion and electronic states across a wide temperature range.

Contribution

It provides new insights into the relationship between ionic motion and electronic conductivity in Li$_4$C$_{60}$, highlighting the role of ionic diffusion in electronic properties.

Findings

Ionic motion influences ESR line shape and conductivity.

A change in activation energy at 125 K correlates with ionic DC conductivity onset.

Li$_4$C$_{60}$ remains metallic in the monomeric phase up to 700 K.

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.