Dispersion relations of the intercalate modes in graphite intercalation compounds

We study the dynamics of the charged intercalate molecules. The model for the intercalate is a two dimensional plasma with the appropriate screening due to the charge carriers in the graphite layers. The dependence of the dispersion relation on stage and charge transfer is discussed and the relation of these modes to conductivity is pointed out.     

Low energy photoelectron spectroscopy on alkali graphite intercalation compounds

The conduction band of various stages of alkali graphite intercalation compounds has been studied by low energy photoelectron spectroscopy (hv ? 6.55 eV). The dissimilar behaviour of the width β of the conduction band peak as a function of photon energy for C₆Li and C₈M (M = K, Rb, and Cs) is discussed in terms of different band types in the vicinity of the Fermi level. The stage dependence of β is measured and interpreted for the system C_xK (for stages 1, 2, 4, and 5).     

Landau levels in graphite intercalation compounds

The first calculation of the magnetic energy level structure of a graphite intercalation compound is presented. The calculational technique exploits the staging symmetry through the k_z-axis zone folding of the magnetic energy levels of the graphite π-bands. The results are applicable to the interpretation of the magnetoreflection and de Haas-van Alphen type experiments in intercalated graphite.     

Raman scattering in graphite intercalation compounds

Raman scattering results are reported on graphite intercalated with Br₂, ICℓ and IBr. In all of these acceptor compounds, the single E_2g2 Raman peak for pure graphite is replaced by a doublet structure identified with in-plane carbon atom vibrations. In addition, Raman peaks specific to the intercalate species are found at frequencies down-shifted from the stretching modes of the free intercalate molecules.     

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.     

Lattice dynamics of graphite intercalation compounds

We apply a k_z-axis zone folding model that accounts for the staging symmetry to calculate the phonon dispersion curves for graphite intercalation compounds of arbitrary stage. The results are applied to calculate the phonon density of states, velocity of sound, and elastic constants for intercalated graphite.     

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.     

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.     

Tuneable sandwich thickness in potassium-ammonia graphite intercalation compounds

We have studied the composition dependence of the sandwich thickness, d_sn, in stages 1 and 2 (n = 1,2) K(NH₃)_xC₂₄ (0<x≤4.48) using high resolution x-ray diffraction. The variation of d_s1 with x is accurately predicted with a model of the sandwich energy which accounts explicitly for charge exchange, f, and for size and stiffness differences between the NH₃ and K species. Surprisingly, the reduction of f with increasing x markedly increases the elastic energy of the K ion.     

Ultrasound-assisted preparation of alkaline graphite intercalation compounds

Alkaline graphite intercalation compounds were prepared by flake graphite, potassium dichromate, concentrated sulfur acid and sodium hydroxide under ultrasound irradiation and characterized by scanning electron microscopy and X-ray diffraction. The influences of solution alkalinity, bath temperature and reaction time on the expansion volume were also investigated. The results show that alkaline graphite intercalation compounds were prepared when ${\text{SO}}_4^{2-}$ and OH^? ions were inserted into the spaces between the graphene planes, producing a flake morphology and multilevel structure. At the same time that the interlayer volume expanded, the oxidizing ability of the solid increased. When the bath temperature, the reaction time and the solution alkalinity were at 33–36 °C, at 60 min and for pH = 13, the top expansion volume was 35 mL g^?1.     

Phenomenological electronic energy bands in graphite intercalation compounds

The electronic energy bands near the Fermi level for both donor and acceptor graphite intercalation compounds are modelled using a k_z-axis zone folded form of the SWMcC bands for pristine graphite. The effect of intercalation is included through terms for the intercalant and for the interaction between intercalant and graphite layers. Graphitic π-bands appropriate to both donor and acceptor compounds are presented.     

Superconductivity of the potassium graphite intercalation compounds

The stratified compound C₄K has been found to pass to a superconducting state with the onset of transition at T_c = 1.3 K.     

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.     

Multilayer model of interlayer spacing in graphite intercalation compounds

The elastic deformation of host layers is calculated by constructing a model where the host layers, regarded as continuous two-dimensional elastic sheets, are infinitely stacked and compressible intercalants, represented by harmonic springs, are intercalated into them. If we treat the intercalants as being rigid relative to the soft host layers in the stage-1 graphite intercalation compound, Li_xC₆ (0≤x≤1), the calculated average gallery spacing agrees well with the experiments, without using any adjustable parameter. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 23 August 2000