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.     

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).     

Observation of selection rules and restricted valence band self convolution for the KVV Auger transition of alkali graphite intercalation compounds

The Auger spectrum of the KVV transition in alkali graphite intercalation compounds (AGIC's) gives unambiguous evidence that the electron states from the alkali metals in the valence band of the compound form a narrow structure in the KVV Auger spectrum which corresponds to the self convolution of these states and that they are not convoluted with the whole valence band. This observation shows that strong matrix element effects or selection rules for KVV Auger transitions have to be taken into account.     

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.     

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.     

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.     

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.     

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.     

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.     

Valence band of molybdenum by photoelectron spectroscopy

Experimental photoelectron spectra of a clean polycrystalline Mo surface excited by monochromatized Al K _α X-rays are presented. The spectra are compared with valence bands obtained by UPS and by band structure calculations within the 5 eV region below the Mo Fermi level. All results mentioned above display peaks at 0.3, 1.7, 2.8 and 4 eV belowE _F. The energy distribution of the valence band does not vary with photon energy and electron emission angle for the four different polycrystalline Mo surfaces compared. It is concluded that the four peaks representing the Mo valence band are predominantly of bulk origin.     

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.     

Thermal properties of graphite intercalation compounds with acids

This work is devoted to systematic thermal analysis of first to fifth stages of binary graphite intercalation compounds (GIC) with HNO₃ and ternary GICs with HNO₃–R (R=CH₃COOH, H₃PO₄) by simultaneous TG–DSC thermoanalyses (Netzsch). Thermolysis of GIC leads to the full deintercalation of acid and significant expansion of samples. The thermal properties of co-intercalated GICs-HNO₃-R and the thermal expanding coefficient of GIC depend on the nature of the intercalate. The endopeak at 110–150 °C corresponds to the loss of nitric acid was observed in DSC- curve of binary second to fifth stages of GICs with HNO₃. A wide endopeak which is characterized by the increasing of decomposition temperature in comparison with GIC-HNO₃ was observed in DSC—curve of ternary GIC with HNO₃–CH₃COOH. Thus, acetic acid stabilizes the graphite nitrate matrix. Two endoeffects due to the deintercalation of HNO₃ (110–180 °C) and H₃PO₄ (450–800 °C) were observed in the DSC-curves of co-intercalated GICs with HNO₃–H₃PO₄ of second and third stages. The value of decomposition heat (ΔH) and weight loss decrease with the increasing of the stage number of GICs. The values of ΔH are equal to 0.40–14.8 kJ/g-at C.