Electrical transport properties of ${\text{Na}}_2{\text{C}}_{60}$ under high pressure

We present direct resistance measurements on ${\text{Na}}_2{\text{C}}_{60}$ under high pressure up to 2 GPa in the temperature range $\sim 90–500\text{K}$. We show that fresh, nominally pure ${\text{Na}}_2{\text{C}}_{60}$ exhibits a semiconductor-type conduction behavior at all experimental conditions, with a 1.2 eV band gap and a conduction strongly influenced by defect gap states. Contrary to ${\text{Rb}}_4{\text{C}}_{60}$ and ${\text{Na}}_4{\text{C}}_{60}$, both band gap and gap states have large pressure dependences. In contrast, a ${\text{Na}}_2{\text{C}}_{60}$ sample preserved in an oxygen-free environment for several months shows conduction by another mechanism, variable range 3D hopping. This is consistent with recent findings of nanoscale phase segregation in ${\text{Na}}_2{\text{C}}_{60}$ and can be understood in terms of conduction by intergrain hopping. Heating the sample at 2 GPa leads to a significant change in the resistivity, suggesting a structural transition. By characterizing the sample after the phase transformation with Raman spectroscopy and x-ray diffraction we identify a phase separation of the material into two main structures, linearly polymerized orthorhombic ${\text{C}}_{60}$ and two-dimensionally polymerized ${\text{Na}}_4{\text{C}}_{60}$.

Figure 1

(Color online) Resistance of ${\text{Na}}_2{\text{C}}_{60}$ (on a logarithmic scale) as a function of temperature at the pressures indicated. Inset shows band gap and localized gap state energy for ${\text{Na}}_2{\text{C}}_{60}$ as functions of pressure as obtained from fitting Eq. (1) to the resistance data.

Figure 2

(Color online) Logarithm of the resistance of phase-separated ${\text{Na}}_2{\text{C}}_{60}$ vs $T^{-1/4}$.

Figure 3

(Color online) (a) Resistance R of ${\text{Na}}_2{\text{C}}_{60}$ versus temperature at 2.0 GPa. Inset shows the conductance $\left(\text{G}=1/R\right)$ versus temperature. (b) Resistance versus temperature at several pressures after heating at 2 GPa. See text for details.

Figure 4

(Color online) X-ray diffraction diagrams for (a) ${\text{Na}}_2{\text{C}}_{60}$ after HPHT, (b) orthorhombic ${\text{C}}_{60}$, (c) ${\text{Na}}_4{\text{C}}_{60}$, and (d) nominally pure ${\text{Na}}_2{\text{C}}_{60}$.

Figure 5

(Color online) Raman spectra of (a) orthorhombic ${\text{C}}_{60}$, (b) ${\text{Na}}_2{\text{C}}_{60}$ after HPHT treatment, (c) nominally pure ${\text{Na}}_2{\text{C}}_{60}$ before the high pressure experiment. Inset shows the $A_g\left(2\right)$ mode positions on an expanded scale.

Figure 6

(Color online) Conductance versus temperature at 0.2 GPa.