Effect of high pressure on electrical transport in the ${\text{Li}}_4{\text{C}}_{60}$ fulleride polymer from 100 to 400 K

In situ resistance measurements have been carried out on ${\text{Li}}_4{\text{C}}_{60}$ 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 ${\text{Li}}^{+}$ ionic conduction with an activation energy near 225 meV. The activation energy decreases with increasing pressure at an initial rate of about $-11\mathrm{\%}/\text{GPa}$ 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 ${\text{Li}}_4{\text{C}}_{60}$ and other alkali-metal intercalated phases such as ${\text{Rb}}_4{\text{C}}_{60}$, ${\text{Na}}_2{\text{C}}_{60}$, and ${\text{Na}}_4{\text{C}}_{60}$. 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.

Figure 1

(Color online) (a) Resistance as a function of pressure for one sample of ${\text{Li}}_4{\text{C}}_{60}$, first for increasing pressure up to 2 GPa at room temperature, then for decreasing pressure after heating at 2 GPa. (b) Data obtained in the same pressure run for ${\text{Na}}_4{\text{C}}_{60}$, with the high-pressure behavior shown on an enlarged scale.

Figure 2

(Color online) The resistance of ${\text{Li}}_4{\text{C}}_{60}$ as a function of temperature between 100 and 300 K at the pressures indicated. Data were taken in the sequence 0.1 and 2 GPa (black), then, after heating to high temperature, at 0.2, 1, and 2 GPa (red, blue, and green curves).

Figure 3

(Color online) (a) Single exponential function $R=A_2\text{exp}\left(E_2/2k_{\text{B}}T\right)$ fitted to the data for the resistance $R\left(T\right)$ to yield $E_2$ in the temperature range 115–295 K and (b) Eq. (1) fitted to the same data set, inverted to the form $G=1/R$. The two fitted energy constants are $E_{\text{g}}=0.45\text{eV}$ and $E_2=0.23\text{eV}$.

Figure 4

(Color online) (a) Fitted excitation energies as functions of pressure. The fitted straight line has a slope $-52\pm 7\text{meV}{\text{GPa}}^{-1}$ and (b) magnitudes of the total conductance and of the conduction terms corresponding to each excitation energy as functions of pressure at 245 K. Circles represent the total conductance, inverse triangles, triangles, and squares the magnitudes of the 0.13, 0.23, and 0.45 eV terms, respectively. The term corresponding to gap states $\left(E_3=0.13\text{eV}\right)$ is very close to zero at 0.1 and 1 GPa and is not plotted at these pressures.

Figure 5

(Color online) (a) X-ray diffraction patterns of ${\text{Li}}_4{\text{C}}_{60}$, (i) at zero pressure before the experiment and (ii) after high pressure and high-temperature treatment. Ticks show line positions for ${\text{Li}}_4{\text{C}}_{60}$ (bottom, green) calculated from literature data (Ref. 6); (b) Raman spectra of ${\text{Li}}_4{\text{C}}_{60}$, (i) at zero pressure before the experiment and (II) after high pressure and high-temperature treatment. Inset shows the $A_{\text{g}}\left(2\right)$ modes.

Figure 6

(Color online) Raman spectra for ${\text{Li}}_4{\text{C}}_{60}$ at the pressures (in gigapascal) indicated.