Intercalation of graphite

The PES measurements arc performed with reference to the Fermi level of the photoclectron spectrometer, in solid specimens, as dealt with here, by the way the spectroscopy works. Thus, in cases when the Fermi level shifts due to some chemical modifications of the sample, i.e., in the intercalation of graphite or other layered compound [16] or in the doping of conjugated polymers 1171, il is necessary to account for the change in the Fermi energy level before interpreting spec-... 

Layer misfit was first recognised in the structures of two metal chloride-graphite intercalation compounds. Intercalates of graphite with many other metal halides, including fluorides and bromides, have since been reported, but they seem to be of the CC type and are not discussed here. 

For the Madelung part of the lattice energy, in the case of a salt for which the structure is accurately known, we employed the method of E. F. Bertaut as modified by D. H. Templeton, and evaluated other terms as set out in Ref. 82. When applied to salts that dissociate easily into gaseous molecules, this provided us, via the vant Hofi relationship (see Refs. 80, 82, and 105) with evaluation of the enthalpies of ionization, AH gg [EFa (g) (EFi i)+(g) -I- F (g)] for a variety of F donor molecules, and the fluoride-ion affinities for well known F acceptors, e.g. Ap(BF3) = — AH29g [BF3(g) + F (g) BF4 (g)]. Such evaluations help to provide a more quantitative evaluation of possible reaction chemistry. The intercalation of graphite by one-electron oxidizers and by fluoride-ion acceptors, as covered in Chap. 10, provides examples. Such evaluations are also useful in assessing the likelihood of the existence of salts not yet known, such as [ArFJ fMFg] (see Ref. 92). 

The discovery of what appears to be a thermodynamic threshold governing the intercalation of graphite by fluoro anions, MF,, has required the evaluation of the thermodynamic stability of a number of such species. Since germanium tetrafluoride and fluorine are intercalated, in combination, by graphite to form both GeFj" and GeFs ", the first and second fluoride ion affinities of that molecule are each of interest. Evaluation of the fluoride ion affinity of boron trifluoride by Altshuller yielded a value of-71 kcal moF. This has been accepted by several authors as the basis for other fluoride ion affinities and electron affinities. Sharpe, however, has preferred a value of -91 kcal moF, based upon the data of Bills and Cotton. Although this latter value is in harmony with other fluoride ion affinities and electron affinities, its confirmation was clearly desirable to provide a firm basis for correction of affinities based upon the lower value. This paper describes the studies that have provided these fluoride ion affinities. 

Intercalation of graphite by arsenic pentafluoride had been reported in 1975, by Lin Chun-Hsu et al. In 1977, E. R. Falardeau et al. reported that graphite fully intercalated by AsFs, had a basal-plane specific conductivity akin to copper metal. I had already pondered on the reasons for the easy intercalation of graphite by AsFs and had wondered if removal of electrons from the 7r-electron band of graphite was involved. 

Whether oxidative intercalation of graphite byfluoroanions can occur may be estimated from the electron affinity, E (-AHm in kcal mol ), of the oxidizing half reaction. For first-stage salts with MFe guests, E must exceed 120 kcal moF . The values of E(e + % MFs MFe + V2 MF3) for AsFs and PFs (125 and 87 kcal mol respectively) account for the spontaneous intercalation of graphite by AsFs and the failure of PFe to do so. Spontaneous intercalation by PFs + Fs occurs because E(e + PFs + V2 F2 PFe ) = 165 kcal... 

Lost of the reactants that bring about intercalation of graphite are either strong oxidants or reductants such as nitric acid or alkali metal (1-3). The oxidation or reduction of the graphite that accompanies the intercalation was clearly shown by Ubbelohde and co-workers (4-6) to be... 

This chapter provides further evidence for the importance of the electron affinity of the oxidizing half reaction in the intercalation of graphite by fluo-rospecies. Moreover, the simple thermodynamic model that accounts for the oxidative intercalation of graphite by fluoroanions is confirmed by reversal of the intercalation in reactions of known enthalpy change. Threshold values of electron affinity (E) for the onset of intercalation and for first-stage intercalation by MFg have also been assessed. 

Figure 1. A thermodynamic cycle for oxidative intercalation of graphite.

Figure 2. Intercalation of graphite hy perfluoroanions as a function of the electron affinity of the oxidizing half reaction.

INTERCALATION OF GRAPHITE BY SILICON TETRAFLUORIDE AND FLUORINE TO YIELD A SECOND-STAGE SALT C24SiFs... 

The pattern of graphite intercalation by hexafluorides of the third transition series [1] had indicated that the intercalation of such molecules, to form Cx MFs salts, requires a minimum electron affinity of approximately 120 kcal mor. Such a threshold electron affinity also accounts [2, 3] for the spontaneous intercalation of graphite by arsenic pentafluoride and for the failure of the physically similar pentafluoride of phosphorus to do likewise. For the half reaction... 

Intercalation of graphite by SiF4 in the presence of F2 has been achieved and a vacuum stable second-stage salt of composition C24SiF5 appears to represent the intercalation limit. This paper describes its preparation and some of its properties. 

Reversible Intercalation of Graphite by Fluorine a New Bifluoride, C12HF2, and Graphite Fluorides, C F (5 > x > 2)... 

R. Hagiwara and N. Bartlett, Thermodynamic Aspects of the Intercalation of Graphite by Fluoroanions, Fluorine-Carbon and Fluoride Carbon Materials, ed. T. Nakajima, Marcel Dekker, Inc., New York (1995) 67-93. 

G. L. Rosenthal, T. Mallouk and N. Bartlett, Intercalation of Graphite by Silicon Tetrafluoride and Fluorine to Yield a Second-Stage Salt C24SiF5, Synthetic Metals 9 (1984) 433-440. 

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