Density functional study of the electronic structure and properties of lithium intercalated graphite

Ab initio electronic-structure calculations are performed using density functional theory (DFT) with polarized basis set (LanL2DZ and 6-311G⁺⁺) within the spin polarized generalized gradient approximation for lithium intercalated graphite. Initially different benzene-Li⁺ model clusters are optimized on the basis of their total energy at room temperature. These model clusters are used to calculate the optimized structure of lithium intercalated graphite clusters. The resultant optimized structures are used to calculate dipole moment, ionization potential (IP), electron affinity (EA), binding energy (BE) and vibrational spectra (IR and Raman). For an idea of the band gap of the clusters in the ground state, the HOMO-LUMO gap (ΔE_g) has been calculated. To compare the electron transfer ability of different clusters, chemical potential (μ), hardness (η) and their ratio for different clusters have also been determined.     

Magnetoreflection study of graphite intercalated with bromine

We report here the first observation of interband Landau level transitions in graphite intercalation compounds. Magnetoreflection measurements are reported for residue compounds of graphite intercalated with up to 1.2 at.% Br. The observed magnetoreflection resonances show evidence for domains in which the electronic band structure and Fermi energy are only slightly dependent on bromine concentration. These conclusions are in general agreement with recently reported de Haas-van Alphen results for these compounds.     

Energy of metal states in intercalated graphite

X-ray photoemission and electron energy loss spectroscopy of carbon and potassium core levels are used to measure the energy distribution of potassium 4s states in KC₈. Contrary to many earlier theoretical calculations these states form a completely empty band with a minimum energy 2.2 eV above the Fermi level in good agreement with the most recent ab initio KKR calculations. Evidence for hybridized states near the Fermi level is presented.     

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.     

Electronic density of first stage lithium intercalated graphite

Electron momentum density of the first stage lithium intercalated graphite, LiC₆, has been investigated by X-rays Compton scattering experiments. It is established that the electron given by lithium to carbons is not delocalized but at the same time is different from a graphite Π electron, as expected in the rigid band model.     

Angle-dependent X-ray emission bands of potassium intercalated graphite

The angular dependent X-ray C K-emission bands of C₈K, C₂₄K and pristine graphite were measured at take-off angles 20° and 80°. For the potassium graphite intercalated compounds (GICs) a pronounced peak is observed at 283.2 eV. Numerical decomposition of the emission bands into their constituent π- and σ-subbands gives evidence for non-rigid band behaviour of potassium GICs and shows that the peak has to be attributed to carbon π-like states.     

Raman scattering in stage 1 europium intercalated graphite

The Raman spectrum of stage 1 europium intercalated graphite, EuC₆, has been recorded at room temperature. EuC₆ has a p(√3 × √3)R30° in-plane superlattice structure, the same as that of LiC₆. The c-axis stacking sequences of EuC₆ and LiC₆ are different. The Raman spectrum of EuC₆ exhibits a broad asymmetric Fano resonance peak at 1500 ± 5 cm^-1, similar to that observed in MC₈ (M = K, RbandCs). However, there is no evidence of any spectral feature around 580 cm^-1 as what has been observed in MC₈. Considering the Raman features of various graphite intercalation compounds in light of their superlattice structures, we suggest that: (1) The feature around 580 cm^-1 is a characteristic associated only with the MC₈p(2 × 2)R0° in-plane superlattice structure. It should originate from the M point phonons of pristine graphite, which become Raman active as predicted by phonon band structure calculations. (2) The continuous background in the spectra of EuC₆ and MC₈, which results in the Fano resonance peak at 1500 cm^-1, is associated with the staggered stacking sequence. Stacking faults are easy to occur in crystals with staggered stacking sequences, but not in crystals with complete elliptical sequences. Therefore, a disorder induced continuum is observed in the Raman spectra of EuC₆ and MC₈, but not in that of LiC₆.     

Orientational epitaxy of an incommensurate neon monolayer adsorbed on graphite

The structure of a monlayer of neon adsorbed on the (0001) surface of a graphite single crystal has been studied at T? 10 K by using high resolution Low Energy Electron Diffraction. The incommensurate monlayer is rotated by ?17° with respect to the (√3 × √3) 30° commensurate structure for a mean overlayer interatomic spacing d? 3.24 Å. Thus result agrees with the prediction of Novaco—McTague theory.     

Quadrupole interactions in MoCl5 intercalated in graphite

The time-differential perturbed angular correlation technique was used to investigate quadrupole interactions following the decay of⁹⁹Mo as a probe in the intercalation compound graphite-molybdenum pentachloride. Analysis of the 740-(44) 141 keV γ-γ correlation in⁹⁹Tc reveals the presence of two sites with static electric field gradient interactions, one of which corresponds to a moderately damped (δ∼16%), high-frequency interaction (v _q∼630 MHz), the other to a heavily damped (δ∼28%), low-frequency (v _q∼283 MHz) component.     

Composition and electronic structure of stage II BF4- intercalated graphite

Graphite was reacted with solid nitryl tetrafluoroborate to form a stage II compound. The experimental de Haas-van Alphen frequencies and cyclotron masses are in agreement with theoretical predictions of the rigid band model with the Fermi energy equal to -0.96 eV. The charge transfer is one elementary charge per 41.5 carbon atoms.     

TDPAD studies of graphite intercalated by AsF5 vapour

The TDPAD technique has been used via the¹⁹F(p,p¹γ)¹⁹F reaction to study the graphite intercalation compound (GIC) formed in AsF₅ vapour. The spectra are dominated by the quadrupole interaction corresponding to the formation of C-F bonds, withv _q~58 MHz, η=0 and δ=0.04. However, peaks are also observed which can be ascribed tentatively to the presence of AsF₃. The strong textural features of the spectra can be related to the direction of the electric field gradient (efg) with respect to the incident beam direction and the detector plane. This suggests that potentially TDPAD can be useful for the characterization of GIC's.     

Electrical properties of Cu-O monolayers intercalated into crystalline graphite

Electrical properties of Cu-O monolayers intercalated into crystalline graphite have been studied at microwave frequencies (up to 1 GHz) in the temperature range 80–400 K. As the temperature increases above 300 K, the resistance of the starting graphite samples increases because of oxygen desorption. Heating a sample containing intercalated Cu-O layers results in a transition from the metallic to the semiconducting (or insulating) state in the 95–130 K interval. At T=8 K, the samples exhibit microwave absorption typical of superconductors (this effect is not observed at T=260 K). Fiz. Tverd. Tela (St. Petersburg) 39, 97–100 (January 1997)     

Elastic plates model of domain walls in intercalated graphite

The recently proposed model of thin elastic plates coupled via harmonic forces for the calculation of domain wall energies in intercalated graphite is extended to higher stages. It is found that whether coherent domains bind to each other depends on the degree of stagger. The effect of the loss of coherence and the interface between different stages are discussed.     

Sinusoidal incommensurate structure in spiral antiferromagnets

It is shown, that in hexagonal antiferromagnets, the form of incommensurate structure depends on the ratio of dipole-dipole energy to the anisotropic interaction energy at fixed values of exchange constants. In CsCuCl₃-type compounds, in which the spiral magnetic structure realizes along the c-axis and 120° structure - in the c-plane, there appears a new sinusoidal phase in the external field H>H_c, where H_c is a critical field. Thus, in result, two modulated structures form at intermediate temperatures.     

Soliton theory for the weakly incommensurate phase of monolayer krypton or graphite

Motivated by a recent computer simulation of the commensurate-incommensurate transition of monolayer krypton on graphite, a theoretical model for the weakly incommensurate phase at finite temperatures is presented. The phonon spectrum of the adsorbed layer and the width of the incommensurate domain walls are calculated selfconsistently. Results for the wall-width are shown to be in good agreement with the findings of the computer simulations.     

Effects of graphite intercalation with cesium in a thermionic converter

The thermionic energy converter (TEC) with inter-electrode low-temperature plasma and cesium vapor dynamic flow in the inter-electrode gap (IEG) demonstrated an increase in efficiency of up to 20%. This was mainly reached due to the decrease in the effective electron work function to an anomalously low value of the order of 1 eV from a perforated nickel collector covered with nanosized graphite flakes under the converter working conditions. SEM X-ray microanalysis of the collector surface layers was performed, and the model of the effects proposed.