Nuclear magnetic resonance study of fluorine-graphite intercalation compounds

To study the origin of semimetal-metal and metal-insulator transformations, localization effects and C-F bonding in fluorine-intercalated graphite C_xF, ¹³C and ¹⁹F NMR investigations have been carried out for a wide range of fluorine content, 3.8 ≤ x ≤ 12.7. Fluorine spectra for small fluorine content, x > 8, are attributed to mobile fluorine acceptor species which are responsible for the increase of electric conductivity in the dilute compound. When increasing the fluorine content to x ∼ 8 corresponding to the maximum electric conductivity, covalent C-F bonds start to occur. The number of these bonds grows with fluorine content resulting in a decrease in conductivity which is caused by a percolation mechanism rather than by a change in bond length. A difference in ¹⁹F chemical shift for fluorine-intercalated graphite C_xF and covalent graphite fluoride (CF)_n has been observed and is attributed to different C_xF bonding in these compounds.