9. New Directions in Li@C₆₀ Research: Physical Measurements

Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy of [Li⁺@C₆₀]\({{\text{PF}}_{6}}^{-}\) and Li@C₆₀ were performed to elucidate their electronic structures. UPS of Li@C₆₀ showed characteristic peaks due to electron transfer from the inner Li to the C₆₀ cage. Dielectric measurement for [Li⁺@C₆₀]\({{\text{PF}}_{6}}^{-}\) at low temperature revealed a phase transition temperature T _C at 24 K. Above T _C, the Li⁺ ion was localized at two equivalent position, suggesting quantum tunneling motion between the positions. Below T _C, these two sites became inequivalent, with occupancies of 76 and 24%, creating electric dipole moments that were canceled by the antiferroelectric configuration of the crystal. Far-IR absorption measurements of [Li⁺@C₆₀]\({{\text{PF}}_{6}}^{-}\) showed terahertz (THz) absorption because of the motion of inner Li⁺. Not only a rotational band (ca. 1.2 THz) at high temperature but also absorption bands due to restriction of Li⁺ motion at low temperature (ca. 2.2 and 2.65 THz) were observed in the THz spectra. The Li atom inside the C₆₀ cage in carbon nanotubes was observed by transmission electron microscopy with electron energy loss spectroscopy.