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 CxF, C-13 and F-19 NMR investigations have been carried out for a wide range of fluorine content, 3.8 less than or equal to x less than or equal to 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 similar to 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 F-19 chemical shift for fluorine-intercalated graphite CxF and covalent graphite fluoride (CF)(n) has been observed and is attributed to different C-F bonding in these compounds.