Terahertz and infrared spectroscopy of the Li@C 60 PF 6 endofullerene

Using terahertz and infrared spectroscopy, we obtained spectra of the complex dielectric permittivity for $^{6,7}\mathrm{Li}$@${\mathrm{C}}_{60}{\mathrm{PF}}_{6}$ and $^{7}\mathrm{Li}$@${\mathrm{C}}_{60}{\mathrm{ClO}}_{4}$ pressed pellets across the frequency range of 10--700 ${\mathrm{cm}}^{\ensuremath{-}1}$ and at temperatures from 5 to 300 K. A rich set of absorption lines was observed, and their origins were analyzed based on detailed temperature dependencies of lines' frequencies, intensities, and damping factors, as well as computer simulations. An isotopic frequency shift was observed for several lines and was considered in the analysis of their origins. At terahertz frequencies, we identified a temperature-dependent absorption band, for which two possible explanations are proposed: quantized rotation of the Li ion within the ${\mathrm{C}}_{60}$ cage and thermally assisted (above $\ensuremath{\approx}100$ K) or tunnel-assisted (below $\ensuremath{\approx}100$ K) hopping of Li over minima in the localizing potential. From the activated behavior of the frequency and intensity of this band, the depths of the corresponding potential wells were estimated to be approximately 4.3 meV above $\ensuremath{\approx}100$ K and approximately 7 meV below $\ensuremath{\approx}100$ K. Additionally, signatures of excitation around 6 ${\mathrm{cm}}^{\ensuremath{-}1}$ were detected, which was attributed to vibrations of the ${\mathrm{C}}_{60}$ lattice. Significant disorder among the $^{7}\mathrm{Li}$@${\mathrm{C}}_{60}{\mathrm{ClO}}_{4}$ molecules is observed, resulted in a broad background absorption over the entire frequency range (10--700 ${\mathrm{cm}}^{\ensuremath{-}1}$) and temperature 5--100 K interval. The results and insights gained from this study could be used in designing nanoelectronic devices based on Li@${\mathrm{C}}_{60}$ molecules.