Surface composition of graphite intercalation compounds

We present Auger spectroscopy measurements on graphite intercalation compounds, for donor (cesium) and for acceptor (MnCl₂) intercalants. The surface composition of both compounds is steeply different from their bulk composition. In the case of high stage cesium compounds, a segregation of the intercalant towards the surface layers is observed, leading to an increased concentration between the first two graphite layers (CsC₈ or CsC₆). On the opposite, an intercalant depletion between these first layers is observed for the first stage (MnCl₂)C₆ compounds.Both effects are explained by the screening of surface states, which involves an electrostatic energy of the same order of magnitude as the intercalant chemical potential. This is thus sufficient to allow the local intercalant concentration to change.     

Self-consistent band structures of higher stage graphite intercalation compounds

Self-consistent non-empirical band structures of third and fourth stage graphite intercalation compounds have been calculated by using the numerical-basis-set LCAO method within the local density functional formalism. The calculations are carried out for a thin film model consisting of n contiguous graphite layers bounded by two partially ionized intercalant layers. The calculated band structures show that most of electrons transferred from the intercalant layers occupy the states in the lowest two conduction π bands mainly localized on the bounding graphite layers. The low-energy optical transitions of higher stage GICs are discussed in terms of the obtained band structures.     

Temperature dependence of C-axis electrical resistivity and thermopower of graphite intercalation compounds

Results on the temperature variation from 2 ? T ? 300 K of the c-axis electrical resistivity and thermopower of a donor stage-5 potassium and an acceptor stage-2 FeCl₃ graphite intercalation compounds are reported. Comparing the data with those of the literature reveals for both compounds a different qualitative behavior for low and high stages.     

Magnetoreflection studies of graphite intercalation compounds

Magnetoreflection spectra are presented for the first time for donor graphite intercalation compounds and for acceptor compounds of low stage. Analysis of these spectra yields values for the K-point effective masses for the conduction and valence bands. Shifts in Fermi level are determined and a breakdown in selection rules for K-point transitions is reported.     

Acceptor site in metal halide intercalated graphite

We present evidence suggesting that metal ion vacancies in the intercalant layer produce acceptor sites responsible for the charge transfer in C_6.6FeCl₃ and related intercalation compounds.     

The de Haas-van Alphen effect of graphite-arsenic pentafluoride interacalation compounds

The de Haas-van Alphen (dHvA) effect in four different samples of graphite-arsenic pentafluoride (AsF₅) intercalation compounds is studied. Each sample with different stage number exhibits different series of dHvA periods. The observed dHvA periods are compared with a model calculation based on the graphite rigid band and supplemented with band folding due to the c-axis superlattice structure of intercalation compounds. From the comparison, the charge transfer from the intercalant to graphite band is estimated to be between 0.2 and 0.3 holes per AsF₅ molecule. It is also found that the charge transfer rate is slightly dependent on the stage number and is smaller for lower stage compounds.     

Conduction electron spin resonance in acceptor-type graphite intercalation compounds

An initial survey of the conduction electron spin resonance is presented for a series of graphite compounds intercalated with acceptor molecules: stages 1–3 AsF₅, stages 2–5 HNO₃, and stage 2 Br₂ and ICl. The g-values and lineshapes were studied as functions of temperature and concentration. The results suggest metallic behavior but with very small density of states at the Fermi energy: N(E_F) ～10²⁰/cm³ eV. The temperature dependence of the linewidth is dominated by an order-disorder transition of the intercalant layers, implying that the conduction electrons are not entirely confined to the graphite portion of the crystal. The decrease in g-value anisotropy upon intercalation can be understood in terms of Elliott's theory.     

Observation of unfilled electron states in alkali graphite intercalation compounds by secondary electron spectroscopy

The energy position of distinct σ-electron energy bands above the Fermi level has been measured in pure graphite, in a variety of stage 1 alkali intercalation compounds and in several stages of C_xK. Changes of the σ-band gap between occupied and unoccupied states near the Λ-point by a nonuniform shift of the valence- and conduction-bands are small for the heavy alkali graphite intercalation compounds, whereas a change of 1 eV is observed for C₆Li.     

Electronic structure of superconducting KC8 and nonsuperconducting LiC6 graphite intercalation compounds: evidence for a graphene-sheet-driven superconducting state

We have performed photoemission studies of the electronic structure in LiC(6) and KC(8), a nonsuperconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC(8) than in LiC(6), opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and superconductivity monotonically increase with the filling of graphene π(*) states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.     

In situ conduction ESR and theoretical studies of graphite intercalation by antimony pentafluoride

The results of an in situ conduction electron spin resonance (CESR) study of the intercalation of SbF₅ molecules into highly oriented pyrolytic graphite are presented. The narrowing (broad-ening) of the CESR signal from intercalated (nonintercalated) parts of the graphite plate during advance of the reaction front into graphite is explained by the nonzero probability of the spin reorientation at collisions of current carriers with the intercalation front and by a decrease (increase) of frequency of these collisions. The assumption was made that the stepwise increase in the intensity of the CESR signal from intercalated parts of the graphite plate during the reaction is due to the presence of an intercalation threshold and periodical impoverishing of the adsorbed layers of the intercalant. The results of calculations conducted within the framework of this model fit the experimental data well. Presented at the 5th Asia-Pasific EPR/ESR Symposium, August 24–27, 2006, Novosibirsk, Russian Federation.     

X-ray characterization of graphite intercalation compounds

The understanding of electronic and lattice properties of graphite intercalation compounds depends critically on the model describing the structural properties. We report here results showing that well-staged as-grown samples do not exhibit the expected in-plane intercalant density, and that careful analysis of the 00? x-ray diffractograms reveals important information on the in-plane occupation probability.     

Evidence for an alkali-like conduction band in alkali graphite intercalation compounds

The valence bands of pure graphite and several alkali graphite intercalation compounds (AGIC's) were studied by UPS (hv = 21.2 eV). The most significant observation is an intensity peak at the Fermi energy E_F in the intecalation compounds. This peak is mainly due to alkali-like s-states. The density of states at E_F is enhanced by a factor of 30 compared to pure graphite. The alkali-like conduction bands in the first stage AGIC's are similar to those of pure alkali metals.     

Alkali metal intercalation in ternary and multinary compounds

Summary The group-IV,-V and-VI transition metal dichalcogenides are able to rapidly and reversibly incorporate significant concentrations of alkali metal ions (intercalants) within their layer structures. Because of their unusual combination of rapid mobility and high chemical stability for the intercalant, some of the alkali-metal-intercalated dichalcogenides are attractive candidates for high-energy battery electrodes. The structural characteristics of the lithium- and sodium-intercalated TiS₂ compounds, which are the most extensively investigated intercalated materials, are summarized. The chemical stability of intercalants is discussed, with an emphasis on the ternary-phase equilibria, electrochemicalcell voltage and intercalant interaction energies. Some recent investigations with multinary alkali metal intercalation compounds are then described. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Infrared and Raman spectroscopy of graphite-ferric chloride

We present the first detailed study of the stage dependence of the IR- and Raman-active optic graphitic modes in a graphite acceptor intercalation compound. The general frequency upshift observed with increasing FeCl₃ concentration for all optic modes is interpreted to indicate an in-plane compression within the graphitic layers. An identification of the IR-active modes with bounding and interior graphite layers is made. A lineshape analysis of the IR spectra implies IR dipole moments corresponding to ～70% of the effective charge in the graphite bounding layers, independent of stage, and ～30% distributed among the graphite interior layers for stage n?3 compounds.     

Volume properties and compressibility of the graphite-potassium-mercury compounds

Abstract

The volume properties of graphite intercalation compounds (GIGS) C4KHg and of the initial intermetallic compounds KHg and KHg₂ have been investigated in the piston-cylinder apparatus, using the direct volumetric technique, under pressures up to 25 kbar. The compounds, average compressibility K₊, was determined to be 3.8×10^?3 kbar^?1 for C₄KHg, 3.0×10^?3 kbar-^?1 for C₈KHg, 4.8×10^?1 kbar^?1 for KHg, and 4.0 ×10^?1 kbar^?1 for KHg₂ at pressures of 0-20 kbar. The compressibility of the “two-dimensional” KHg layer in the GIC under various pressure conditions has been estimated. These estimates permit comparison of KHg properties in the “three dimensional” and “quasi-two dimensional” states. It was concluded that the influence of the graphite matrix on the intercalant is insignificant for this type ternary GIC.     

Island formation in metal halide intercalation compouds

Acceptor action, non-stoichiometry, and island formation of metal halide intercalants in graphite are explained by the tendency toward full six-fold halogen coordination at the boundary of the intercalant. The Fermi level is pinned on the acceptor level split-off from the halogen p-derived valence band.     

The effect of disorder on weak localization and electron-electron interaction in low stage graphite intercalation compounds

New high resolution temperature-dependent electrical resistivity data on low stage acceptor graphite fiber intercalation compounds are presented. A logarithmic increase of the resistivity with decreasing temperature and a negative magnetoresistance are observed in the low temperature range. It is shown that the magnitude of the effect and the temperature of the resistivity minimum are directly related to that of the residual resistivity of the compound. All the results are consistent with a two-dimensional weak localization and electron-electron interaction model. It is suggested that graphite intercalation compounds are the most appropriate systems to investigate 2D localization and interaction effects.     

Intercalation of graphite and hexagonal boron nitride by lithium

Although graphite and hexagonal form of BN (h-BN) are isoelectronic and have very similar lattice structures, it has been very difficult to intercalate h-BN while there are hundreds of intercalation compounds of graphite. We have done a comparative first principles investigation of lithium intercalation of graphite and hexagonal boron nitride to provide clues for the difficulty of h-BN intercalation. In particular lattice structure, cohesive energy, formation enthalpy, charge transfer and electronic structure of both intercalation compounds are calculated in the density functional theory framework with local density approximation to the exchange-correlation energy. The calculated formation enthalpy of the considered forms of Li intercalated h-BN is found to be positive which rules out h-BN intercalation without externally supplied energy. Also, the Li(BN)₃ form of Li-intercalated h-BN is found to have a large electronic density of states at the Fermi level and an interlayer state that crosses Fermi level at the zone center; these properties make it an interesting material to investigate the role of interlayer states in the superconductivity of alkali intercalated layered structures. The most pronounced change in the charge distribution of the intercalated compounds is found to be charge transfer from the planar σ states to the π states.     

The de Haas-van Alphen effect of graphite intercalation compounds with SbCl5 and HNO3

The de Haas-van Alphen (dHvA) effect of SbCl₅-graphite intercalation compounds of stage 2, 3 and 4, and residual HNO₃-compound of stage 3 has been studied. The dHvA spectra are stage dependent, and no combination frequency relations are found, which are in disagreement with Batallan et al.'s report. The amount of charge transfer per intercalant estimated on the basis of the rigid band model is 0.44, 0.49 and 0.43 for stage 4, 3 and 2 SbCl₅-compounds and is 0.14 for stage 3 HNO₃-compound.