Abstract
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.
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Matsuo, Y., Okada, H., Ueno, H. (2017). New Directions in Li@C60 Research: Physical Measurements. In: Endohedral Lithium-containing Fullerenes. Springer, Singapore. https://doi.org/10.1007/978-981-10-5004-6_9
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DOI: https://doi.org/10.1007/978-981-10-5004-6_9
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