STRUCTURAL AND ELECTRONIC PROPERTIES OF LITHIUM DOPED FULLERIDES

摘要

The small Li ion dimension makes the lithium intercalated fullerides Li_xC_(60) display quite different properties as compared with other better known alkali doped fullerides. The most relevant are a large doping range (x = 1-30), the lack of superconductivity and the formation of polymerised C_(60) structures, especially observed at low lithium concentrations. At high doping levels (x ～ 15) both diffraction (1) and multiple-quantum NMR (2) clearly show the formation of lithium clusters intercalated in the largest interstices of the C_(60) lattice. On decreasing the lithium concentration, C_(60) units begin to polymerise (usually in a 2D network), as shown by X-ray diffraction, Raman spectroscopy and confirmed also by ~(13)C NMR lineshape analysis, which indicates the presence of sp~3 carbons. Structural and vibrational properties of Li_4C_(60) have been investigated in detail by x-ray powder diffraction, ~(13)C NMR and Raman spectroscopy. The polymer nature of this insulating fulleride is indicated by all the employed techniques. The charge transfer to C_(60) as related to the shift of the A_g(2) mode is discussed in detail.