A MODEL FOR DISORDER IN FLUORINE-INTERCALATED GRAPHITE

The structural and electronic properties of fluorine- and bromine-inte rcalated graphite fibers and HOPG are summarized. In contrast to the b romine intercalate, which is purely ionic for any experimentally attai nable intercalate concentration, fluorine has a dual ionic and covalen t behavior in graphite. Furthermore, whereas bromine-intercalated grap hite is ordered, fluorine-intercalated graphite is disordered. The sti ff graphene planes are buckled and islands of various fluorine concent rations are formed. A thermodynamic model is proposed that accounts fo r the differences between fluorine- and bromine-intercalated graphite materials. The model describes the competition between ionically bonde d and covalently bonded intercalate phases of fluorine in graphite. Co valent bonding is more favorable energetically, but an important nucle ation barrier exists due to strain and to the destruction of the conju gation of the double bonds.