Alkali Graphite Intercalation Compounds Research Articles

層間化合物とその物性 : グラファイト・アルカリ金属層間化合物を中心として

層間化合物とその物性 : グラファイト・アルカリ金属層間化合物を中心として

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).

Statics and dynamics of interlayer interactions in the dense high-pressure graphite compoundLiC2

Reaction of Li with graphite at high pressures yields ${\mathrm{LiC}}_{2}$, three times as dense in Li than the ambient-pressure graphite intercalation compound ${\mathrm{LiC}}_{6}$. We study the stability of this unusually high Li density against Coulomb repulsion by neutron and x-ray scattering. Surprisingly, parameters which should be sensitive to interlayer interactions are quite similar to the ${\mathrm{LiC}}_{6}$ values: c-axis compressibility ${\ensuremath{\kappa}}_{c}=1.43\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12} {\mathrm{cm}}^{2}/\mathrm{dyn}$, sound velocity ${v}_{s}=5.1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm/s, zone-boundary acoustic-phonon energy = 18.5 eV, and slightly higher thermal expansion ${\ensuremath{\alpha}}_{c}=66\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$/K (300--450 K). Moreover, both compounds are yellow in reflection, implying that the delocalized charge densities are comparable. These and other results are consistent with partially covalent in-plane Li-Li bonds, i.e., partial charge transfer to the graphene layers, as opposed to the more conventional ionic picture which applies to most alkali-graphite intercalation compounds.

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.

Polarization dependence of the infrared-active modes of pyrolytic graphite (HOPG) and a potassium-HOPG intercalation compound

The infrared-active modes of HOPG have been measured under ultrahigh vacuum conditions. The two vibrational bands have been found to be strongly dependent on the polarization of the incoming IR-beam. Under s-polarization the A 2u-mode is not detectable. After evaporation of potassium onto the cooled HOPG sample the spectra show bands in the vicinity of the E 1u-mode. These bands are characteristic for alkali-graphite intercalation compounds. Preliminary measurements in the far infrared region show a broad and intense band centered at about 270 cm −1.

U.P.S. and X.P.S. studies of alkali-graphite intercalation compounds

K, Rb and Cs graphite intercalation compounds have been studied by photoelectron spectroscopy both in U.P.S. and in X.P.S. The study in high vacuo purity conditions and on well characterized samples ensures the experimental results to be relative to clean and not deteriorated surfaces. This certainty has been verified by comparison with spectra obtained on the same surfaces after a well-controlled oxidization. Core and valence spectra of carbon and alkaline species obtained at three different energies (1253.6, 21.2 and 40.8 eV) at room and at low temperature are analyzed in terms of stoichiometry and surface cleanness. Observing the surface of the first stage Cs-, Rb- and K- graphite intercalation compounds, the XC 8 composition has been obtained provided the bulk is stoichiometric and the ultra-high vacuo of good quality. Criteria for surface cleanness are described and the work functions given.

Electronic structure of disordered ternary alkali-graphite intercalation compounds

The density-of-states for the π and the metal/interlayer bands for K xCs 1−xC 8 has been calculated as a function of composition using both the virtual crystal and the average t-matrix approximations. The results show a broadening and, for some compositions, a splitting of the sharp peaks in the valence and conduction band densities-of-states.

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.

Structural Energies in Stage-One Graphite Intercalation Compounds

A Thomas-Fermi density-functional theory is developed for structural energies in stage-one alkali-graphite intercalation compounds. Correct trends for lattice constants and elastic moduli are obtained. We compute corrugation energies $\ensuremath{\Delta}E$ and alkali-alkali spring constants $k$, which determine in-plane intercalant diffusion and domain-wall structure. $k$ is determined by electrostatic effects, and is mainly independent of intercalant. $\ensuremath{\Delta}E$ depends strongly on intercalant size; thus Li differs substantially from K, Rb, and Cs.

Structural Energies in Stage-One Graphite Intercalation Compounds

A Thomas-Fermi density-functional theory is developed for structural energies in stage-one alkali-graphite intercalation compounds. Correct trends for lattice constants and elastic moduli are obtained. We compute corrugation energies $\ensuremath{\Delta}E$ and alkali-alkali spring constants $k$, which determine in-plane intercalant diffusion and domain-wall structure. $k$ is determined by electrostatic effects, and is mainly independent of intercalant. $\ensuremath{\Delta}E$ depends strongly on intercalant size; thus Li differs substantially from K, Rb, and Cs.

Ab InitioCalculation of the Optical Spectra of LiC6and the Origins of its Plasmons

We present the results of the first ab initio calculation of the optical spectra of Li${\mathrm{C}}_{6}$ up to 40 eV. The results are in excellent agreement with experiment and are used to identify the structures seen in the optical spectra of this compound, and are also used to clarify the origins of the observed and calculated plasmons. Some of the results appear to be common to all alkali-graphite intercalation compounds.

Positron annihilation studies of stage-1 donor graphite intercalation compounds

Measurements of the 2γ-Angular Correlation Distribution (ACD) of the positron annihilation radiation in the stage-1 donor graphite intercalation compounds C 6Li, C 6Ba, C 8K, C 8Rb and C 8Cs are presented. The ACDs of C 6Li are found to be very similar to those of graphite, indicating that annihilations preferentially occur with graphitic σ-and π-electrons. By contrast, in the three heavy alkali metal compounds, an intense, strongly anisotropic, narrow component dominates the ACDs. This can be attributed to annihilations with electrons exhibiting a free-electron character parallel to the layers. The anisotropy decreases with increasing atomic number of the intercalant. This observation is related to the expansion of the distance between the carbon planes upon exchanging the intercalant. The ACDs of C 6Ba are found to be similar to those of the heavy alkali-graphite intercalation compounds; the anisotropy of the ACDs, however, is very small. Comparison with C 8K indicates that the narrow component of the ACDs in C 6Ba cannot be attributed solely to interlayer states exhibiting free-electron character parallel to the layers but rather to Ba metal states.

Electronic structure of C 6Ba studied by photoelectron spectroscopy

We have prepared stage 1 barium-graphite intercalation compounds by vapour phase reaction, and a new type of liquid phase reaction, whereby homogeneous large samples are obtained. The electronic structure of these samples has been studied by XPS and UPS. The measurements show strong similarities to the alkali-graphite intercalation compounds, but the conduction band seems to have a more complex structure with both graphite derived and barium derived bands. The C 1s core level binding energy shift indicates a charge redistribution towards the carbon site which is similar to that in C 6Li.

Thermal expansion and lattice anharmonicity of alkali-graphite intercalation compounds

We have measured the $c$-axis and $a$-axis thermal expansion of several alkalimetal---graphite intercalation compounds by means of x-ray scattering between 10 and 300 K. The $c$-axis thermal expansion depends strongly on the in-plane mass density of the alkali-metal atoms and the stage of the compound, and is largest for K${\mathrm{C}}_{8}$. The $a$-axis thermal expansion of K${\mathrm{C}}_{8}$ and K${\mathrm{C}}_{24}$ is negative in the considered temperature region and the room-temperature coefficient is twice as large as for pristine graphite. We have analyzed the $c$-axis data in terms of a one-dimensional quasiharmonic approximation, which yields Gr\uneisen parameters 2 or 3 times as large as those for graphite.

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.

Theory of superconductivity in graphite intercalation compounds

The superconducting transition temperature, T c, of the first stage alkali-graphite intercalation compounds (GICs), especially of C 8K, has been calculated, assuming that the three-dimensional (3D) alkali-like electrons contribute to superconductivity. We suggest that the McMillan formula for T c is applicable to the GIC systems, in contrast to a previous theoretical treatment by Takada. Thus the mass enhancement factor of the 3D electrons due to the electron-phonon interactions is calculated from the first principle. It is confirmed that the interaction with the longitudinal phonon modes in which the carbon and alkali metal atoms vibrate along the c-axis is very strong, while that with the phonon modes in which all the atoms vibrate along the layers is screened by the two-dimensional, graphite-like electrons. The former mechanism is characteristic of GICs and this results in superconductivity in GICs. The latter, i.e., the screening effect due to the 2D electrons, is mainly responsible for the reduction of T c by pressure, recently observed in alkali-amalgam GICs.

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.