Alkali Graphite Research Articles

Synthesis of Electroconducting Hydroxy-Sodalite/Graphite Composite: Preparation and Characterization

Electroconductive hydroxy-sodalite/graphite composites were synthesized by alkali-activation of kaolinite in the presence of sodium hydroxide as the alkali activator and graphite as a conductive filler. Thermal, morphological and microstructural properties in addition to direct current (D.C.) conductivity of the prepared composites were investigated. Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy in the attenuated total reflection mode (FTIR/ATR), X-ray diffraction (XRD), scanning electron microscope/energy dispersive using X-ray analysis (SEM/EDX) and DC conductivity measurements were used to characterize the prepared composites. The effect of the hydroxyl-sodalite-to-graphite and NaOH-to-kaolinite ratios on the electrical conductivity was investigated and evaluated on the generated composite specimens made of Jordanian kaolinite or pure kaolinite. It was demonstrated that increasing the mass ratio of graphite-to-kaolinite in the clay-based composites increased the electrical conductivity of the resultant composites. It was also observed that using 1:1 graphite-to-pure kaolinite mass ratio showed the best electrical conductivity value of 3 × 10-3 s/cm, among the other mass ratios used for pure kaolinite specimens, while using 1:1 mass ratio of graphite-to-Jordanian kaolinite showed a conductivity of 1.6 s/cm.

Photoemission study of K on graphite

The physical and electronic structure of the dispersed and $(2\ifmmode\times\else\texttimes\fi{}2)$ phases of K/graphite have been characterized by valence and core-level photoemission. Charge transfer from K to graphite is found to occur at all coverages, and includes transfer of charge to the second graphite layer. A rigid band description is reasonably successful in describing important aspects of the data, and our results are consistent with a shift of approximately $0.4 \mathrm{eV}$ in the surface graphite layer for both phases. The C $1s$ line shape and binding-energy shift as a function of charge transfer can be understood qualitatively by taking into account rigid band effects and the effects of a core hole on the density of states. For the $(2\ifmmode\times\else\texttimes\fi{}2)$ phase the metallic overlayer contributes extrinsic satellites to the C $1s$ line shape. The K $3p$ spectrum is strongly affected by the overlayer phase, and in addition indicates very little variation in the substrate charge distribution as a function of coverage in the dispersed phase. The lack of an interface K $3p$ binding-energy shift for a K bilayer or multilayer is ascribed to a weak K-graphite bond for metallic overlayers. The results have implications for the interpretation of photoelectron spectra of alkali graphite intercalation compounds (GIC's).

Ab initio study of structural properties of stage‐1 alkali graphite intercalation compounds

AbstractFirst‐principles electronic structure calculations are carried out for the stage‐1 alkali graphite intercalation compounds LiC6 and XC8 (X = Li, Na, K, Rb and Cs). Lattice constants and atomic positions are determined by total energy minimization. It is demonstrated that the experimentally observed AA stacking order of the graphite host is partly a consequence of the electronic charge transfer from the alkali atoms to the graphite host.

Charge density and charge transfer in stage-1 alkali-graphite intercalation compounds

First-principles electronic structure calculations are carried out for the stage-1 alkali graphite intercalation compounds ${\mathrm{LiC}}_{6}$ and ${\mathrm{XC}}_{8}$ (X=Li, Na, K, Rb, and Cs). We analyze the charge densities and the differences to the reference charge densities of graphite host and intercalant sublattice. For the alkali metals Na, K, Rb, and Cs the computed charge transfer is nearly constant at a value of 0.7 elementary charges (e); values of 0.5e and 0.4e are found for the Li compounds ${\mathrm{LiC}}_{6}$ and ${\mathrm{LiC}}_{8}$, respectively. It is shown that the main fraction, about 0.4e, of the charge transfer is a geometrical consequence of the simple overlap of the charge densities of the graphite and intercalant sublattices.

Lattice dynamics of alkali graphite intercalation compounds

A two body potential proposed to account for the interatomic interactions of graphite has been used to investigate the phonon dispersion and the shear elastic constants of first, second and third stage graphite intercalation compounds of lithium, potassium, rubidium and cesium. A fairly good agreement has been obtained with the available experimental results establishing the validity of the two body interatomic potential.

Structure and synthesis of the ternary alkali graphite intercalation compound KCsC16, an ideal layered heterostructure

We have prepared a stable two phase graphite intercalation compound (GIC) containing KC8 and KCsC16 by immersing CsC24 into liquid K at 70 °C. High resolution x-ray diffraction studies reveal that the KCsC16, which exhibits a stage one c-axis stacking sequence of ⋅⋅⋅ C K C Cs C K C Cs ⋅⋅⋅ is an ideal ternary heterostructure GIC with a c-axis repeat distance of dH=11.27 Å and correlation range of 350 Å. The intensities of the (00l) x-ray diffraction pattern calculated on the basis of the proposed c-axis repeat sequence are in excellent agreement with the experimental observations. The KCsC16 in plane structure is a (2×2)R0 ° superlattice with an in plane correlation range of 140 Å. The ordered intercalant layers stack along the c axis with an α,β,γ,δ site sequence, where, e.g., α,γ=K and β,δ=Cs, and stacking faults occur on average every 30 Å.

Interlayer Stacking Structure in First Stage Alkali Binary and Ternary Graphite Intercalation Compounds

Order-order and/or order-disorder transitions associated with changes of intercalate interlayer stacking sequences in stage-1 alkali graphite intercalation compounds are studied within a framework of mean-field lattice gas model. The species dependence of the interlayer stacking phases and phase transitions in binary compounds MC 8 (M=K, Rb, Cs) as well as in ternary compound KCsC 16 is described in terms of the present lattice gas model.

Bragg-reflection profiles of graphite and alkali–graphite intercalation compounds: Comparison of double-axis and triple-axis spectrometer results

Double-axis x-ray spectrometer measurements at flat, low-absorbing mosaic crystals produce broad and asymmetric Bragg-reflection profiles. If the absorption coefficient changes during the sample preparation process, as in the case of graphite and graphite intercalation compounds, a determination of the instrumental profile is almost impossible. Using triple-axis spectrometers, the Bragg-reflection profiles are independent of the sample transparency, and peak profiles can be determined with high resolution and minor corrections.

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 C 6Li and C 8 M ( 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 x K (for stages 1, 2, 4, and 5).

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 x K. 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 C 6Li.

XPS Study of Alkali Graphite Intercalation Compounds

ABSTRACTCarbon 1s X-ray photoemission spectra of alkali graphite intercalation compounds are shown to provide detailed information about the distribution of charge among the carbon layers and atoms. The interior layers of higher stage compounds contain little charge and remain similar to graphite. The charge in the bounding layers is strongly localized to screen the alkali intercalate ions. The c-axis charge distribution in KC12n is in reasonable accord with the electrostatic screening model of Safran and Hamann.

Raman scattering from alkali graphite intercalation compounds

AbstractThe stage dependences of the Raman spectra of potassium, rubidium and cesium graphite intercalation compounds are reported. The spectra of each set of compounds are found to be qualitatively and quantitatively consistent with the ‘nearest layer’ model for the carbon intralayer E vibrations of intercalation compounds. To within experimental error the charge exchanged from the intercalant layers to the carbon layers is found to be stage independent and partially localized in type B carbon layers which are contiguous to intercalant layers. The relative Raman cross‐section for scattering from type B and type C (noncontiguous) carbon layers is found to be species independent and is σC/σB = 0.25 ± 0.05.

Evidence for an Alkalilike Conduction Band in Alkali Graphite Intercalation Compounds.

Evidence for an Alkalilike Conduction Band in Alkali Graphite Intercalation Compounds.

Evidence for an Alkalilike Conduction Band in Alkali Graphite Intercalation Compounds

The valence bands of pure graphite and several alkali graphite intercalation compounds were studied by ultraviolet photoelectron spectroscopy ($h\ensuremath{\nu}=21.2$ eV). The most significant observation is an intensity peak at the Fermi energy ${E}_{\mathrm{F}}$ in the intercalation compounds. This peak is mainly due to alkalilike $s$ states. The density of states at ${E}_{\mathrm{F}}$ is enhanced by a factor of 30 compared with pure graphite. The alkalilike conduction bands in the first stage alkali graphite interclation compounds are similar to those of pure alkali metals.

Interlayer screening and magnetic susceptibility of graphite intercalation compounds

The observed magnetic susceptibility ( x c ) of alkali graphite intercalation compounds (gic) for H parallel to the hexagonal c -axis, is paramagnetic . In K gic the molar value of x c varies rapidly with stage ( n ), increasing by an order of magnitude from C 8 K ( n = 1) to C 48 K ( n = 4). Calculations of x c as a function of Fermi energy (μ) for a single layer model of gic are presented and indicate that the orbital contribution ( X c or ) to X c is paramagnetic for μ ≳ 4 kT in contrast to the usual Landau-Peierls diamagnetism. Furthermore, X c or is shown to be a sensitive function of the conduction electron charge distribution from layer to layer for n ≥3, while the spin (Pauli) contribution to X c and the specific heat coefficient are less sensitive to this distribution. These results suggest that measurements of the stage dependence of X c can be used to estimate the c -axis screening length in gic.

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.

Electronic properties of graphite intercalation compounds

New preparation techniques for Li- and the high stages of the heavy Alkali Graphite Intercalation Compounds (AGIC's) were developed. A new electrochromic effect based on electrochemical intercalation of graphite electrodes was discovered. Electron spectroscopy studies such as XPS, UPS and AES on AGIC's revealed a drastic increase of the density of states and alkali-like s-type conduction bands at the Fermi energy. The shift at EF appears to be almost independent of the intercalant concentration. From optical reflectivity experiments we observe interband transitions to disappear with the ordering of the intercalant layer. The resistivity and magnetic susceptibility measurements indicate no significant difference between a dilute and a concentrated regime.

Gas absorption by lamellar compounds

Abstract K. Tamaru in Japan, P. Lagrange in France previously reported on molecular sieving properties of graphite compounds intercalated with alkali metals. We have shown that the first stage compound ZrS 2 Cs 0.37 reversibly takes up hydrogen and deuterium at low temperature. The amount of sorbed gas corresponds to an H 2 /Cs ratio of 1.3. Unlike alkali graphite compounds, here the sites are of various energy and geometry, and in this way they account for the variations of the thermodynamic and kinetic data. Close-packing of some metallic atoms is probably responsible for the low value of this ratio. Accordingly, a model of the structure of the intercalated atoms is proposed, which is supported by very weak superstructure reflexions.

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.

Radiation Effects in Solids

The theory of radiation effects in solids is summarized, and the various phenomena are illustrated with examples. The effects produced by radiation are electronic excitation, elastic collisions, displacements, inelastic collisions, and transmutations. The eIfects on alkali halides, alpha brass, semiconductors, graphite, and uranium are specifically discussed. (J.S.R.)