Effect of high pressure on electrical transport in the Li4C60 fulleride polymer from 100 to 400 K

In situ resistance measurements have been carried out on Li4C60 under pressures up to 2 GPa at temperatures from below 100 to 400 K. In agreement with recent reports we find an Arrhenius law behavior for the conductivity, which can be interpreted in terms of Li+ ionic conduction with an activation energy near 225 meV. The activation energy decreases with increasing pressure at an initial rate of about -11%/GPa and the room-temperature conductivity increases by a factor of about 6 from 0.1 to 2 GPa. We also observe conductivity terms with a lower excitation energy, most probably associated with conduction by electrons excited from defect-induced states in the main band gap. We discuss this conduction behavior in the context of recent measurements on both Li4C60 and other alkali-metal intercalated phases such as Rb4C60, Na2C60, and Na4C60. After heating to 400 K at 2 GPa the conduction behavior changes drastically, manifested by a change in the slopes of R versus T curves signifying newly created gap states. Postexperimental characterization by Raman spectroscopy and x-ray diffraction indicate the loss of Li especially from the grain surfaces. Finally, high-pressure Raman studies suggest a possible metallization transition above 9 GPa.