Unusual polymerization in the ${\mathrm{Li}}_4{\mathrm{C}}_{60}$ fulleride

${\mathrm{Li}}_4{\mathrm{C}}_{60}$, one of the best representatives of lithium intercalated fullerides, features an unusual type of two-dimensional polymerization. Extensive investigations, including laboratory x-ray and synchrotron radiation diffraction, ${}^{13}\mathrm{C}$ NMR, magic angle spinning, and Raman spectroscopy, show a monoclinic $I2∕m$ structure, characterized by chains of $[2+2]$-cycloaddicted fullerenes, sideways connected by single $\mathrm{C}–\mathrm{C}$ bonds. This leads to the formation of polymeric layers, whose insulating nature, deduced from the NMR and Raman spectra, denotes the complete localization of the electrons involved in the covalent bonds.

Figure 1

Room temperature ${\mathrm{Li}}_x{\mathrm{C}}_{60}$ powder diffraction patterns taken with Cu $K\alpha$ laboratory x-rays for $1⩽x⩽6$. For $x=1,2,6$ there is a clear phase coexistence (bold lines), whereas for $3⩽x⩽5$ the samples are homogeneous. For comparison the pristine ${\mathrm{C}}_{60}$ diffraction profile and its main peak positions (dashed lines) are also shown.

Figure 2

Measured (o) and calculated (solid line) diffraction pattern of ${\mathrm{Li}}_4{\mathrm{C}}_{60}$ at $300\mathrm{K}$. The lower solid line shows the difference profile and the ticks mark the reflection positions. Inset: the polymeric structure of ${\mathrm{Li}}_4{\mathrm{C}}_{60}$.

Figure 3

Fit residuals $R_{\mathrm{wp}}$ as fullerene chains rotate along the $b$ axis. The minimum at $\sim 100°$ corresponds to neighboring C atoms facing each other and giving rise to single bond polymerization (see inset).

Figure 4

${}^{13}\mathrm{C}$ static NMR spectrum of ${\mathrm{Li}}_4{\mathrm{C}}_{60}$ at room temperature along with the global fit and its two components (a Gaussian function and a chemical shielding powder pattern). The narrow peak of pristine ${\mathrm{C}}_{60}$ at $143\mathrm{ppm}$ is also shown.

Figure 5

${}^{13}\mathrm{C}$ MAS $\left(8\mathrm{kHz}\right)$ spectrum at $300\mathrm{K}$ showing the different intensities for $s{p}^2$ and $s{p}^3$ carbon atoms. Inset: the two peaks in the $s{p}^3$ region arise from two different types of ${\mathrm{C}}_{60}$ polymerization.

Figure 6

Room temperature micro-Raman spectra of ${\mathrm{Li}}_4{\mathrm{C}}_{60}$ in a parallel polarization scattering geometry and for two laser excitation energies: 2.41 and $1.96\mathrm{eV}$. The two peaks arising from carbon polymerization are shown in the boxed area.