Graphite Acceptor Compounds Research Articles

Resistivity and low-field magnetoresistance of MNCL 2-GIC

The four-probe method was used to measure the electrical resistivity and transverse magnetoresistance of stage-2 MnCl 2GIC. Both the in-plane and c-axis resistivity were measured in the temperature range between 298 and 12 K. The in-plane resistivity was found to be metallic. This behavior was observed for most GICs. The c-axis resistivity was found to decrease with decreasing temperature, which is a typical behavior for low stages of acceptor compounds of graphite. The in-plane and along the c-axis transverse magnetoresistance were measured in the low magnetic field limit (up to 5 kG) at temperatures 298, 77 and 12 K. The average mobility was calculated from the data of the magnetoresistance assuming a simple two-band model.

Synthesis and physical characterization of new d-transition metal fluoride GICSs

Several routes have been carried out to prepare new GICs containing transition metal fluorides : the gas-solid reaction is employed with gaseous RhF 5 for instance, whereas the liquid-solid reaction in anhydrous hydrogen fluoride saturated with fluoride ions is used with Cr, Rh and Au fluorides. The obtained compounds crystallize in stage-1 to stage-3 depending on the nature of host graphite. The thickness of the intercalate, which can be deducted from the repeat distance (I c) value, generally ranges from 4.50 to 4.85 Å. On the basis of optical reflectivity experiments carried out at Kyoto University, a quantitative interpretation has been proposed using the 2-D band structure model of graphite acceptor compounds (Blinowsky-Rigaux model) [1]. For Cr- and Rh-based materials, the best fitting is obtained for the following Fermi level (E F) and relaxation time (τ): C 21CrF 4.5 E F = 0.94 ± 0.03 eV; τ = 2.8·10 −14 S C 28RhF 3.3(HF 2) 1.3 EF = 0.89 ± 0.03 eV; τ = 1.2·10 −14 S The corresponding charge transfer coefficients can be deduced from the E F values: f c = 0.024 and f c = 0.021), respectively. These values can be correlated to a C-C bond length contraction: • Cr-based GIC: d c-c = 1.4165 Å → 4 x 10 −3 Å contraction • Rh-based GIC: d c-c = 1.4176 Å → 3.3 x 10 −3 Å contraction. This trend is confirmed by the shift towards higher energies observed in Raman E 2 2g peak. The oxidation state of the transition metals has been deduced from ESR, XANES, magnetic and XPS studies, whereas the coordination of Rh and Au has been shown to be octahedral and square planar respectively, using EXAFS spectroscopy.

Magnetoresistance of low stage graphite acceptor compounds

The angular dependences of magnetoresistance (ADM) Δ P P o versus the angle θ between the magnetic field direction and the sample c-axis of the graphite intercalation compound (GIC) are calculated. Comparison of the obtained theoretical dependences with the experimental data is carried out for the stage-2 C 10CuCl 2 and C 16ICl compounds, and also for the stage-I heterointercalated compound C 10CuCl 2·0.6 ICl. It is shown that three dimensional model with the c-axis dispersion of the current carriers is in much better correspondence with the experimental data than two dimensional one.

A method in the study of band structure for GAC

A simple but self-consistent scheme is proposed for studying the electronic structure of graphite acceptor compounds (GAC). The basic concept of the computor renormalization group (CRG) is applied in simplifying the interactions between graphite and the intercalants, and a “pseudo-atom” is used to simulate the intercalant molecule. This model enables us to study different IC 8 type GAC in the same program, the extended Hückel energy band method is developed and the resulting band structures and Fermi surface properties of GAC (FeCl 3, AsF 5) thus obtained are discussed and compared with the present data.

126. Graphite-acceptor compounds as oxidizing agents: reduction of the graphite lattice

126. Graphite-acceptor compounds as oxidizing agents: reduction of the graphite lattice

105. Magnetoresistance of acceptor compounds of graphite

105. Magnetoresistance of acceptor compounds of graphite

117. Diamagnetism of dilute acceptor compounds of graphite

117. Diamagnetism of dilute acceptor compounds of graphite

C-axis charge distribution in stage-3 and -4 graphite acceptor compounds

By generalizing the acceptor model previously developed for stage-2 graphite intercalation compounds, we show that the principal Fermi surfaces of stage-3 and -4 compounds can be described in terms of a single extra parameter. This parameter is directly related to $c$-axis screening of the intercalant layer potential, and can be used to determine how much charge is transferred to the interior layers. We find that the interior-layer charge transfer is similar for all intercalants and stages. Screening effects, while substantial, are weaker than most theoretical predictions.

Diamagnetism of Dilute Acceptor Compounds of Graphite

Diamagnetism study on graphite-acceptor compounds has been further extended over dilute nitrates and slightly neutron-irradiated graphite. An analysis was made on the basis of Sharma-Johnson-McClure's theory taking into account the trigonal warping of the Fermi surface. The susceptibility vs reciprocal termperature plots, including those for IC l compounds not completely interpreted in the previous study, have been theoretically reproduced by assuming a mixed phase structure of orderly intercalated and purely graphitic domains in each sample. The hole production rate (ionization efficiency) of acceptors was evaluated by using Ono-Sugihara's formulae which take into account the trigonal warping effect on the carrier density. It has been found that 5% of intercalated IC l is ionized in the doped regions irrespectively of the total content.

Electronic Transport Properties of Graphite Acceptor Compounds

ABSTRACTCalculations of the magnetoresistance of graphite acceptor compounds are made using a tight binding model for the carrier dispersion proposed by Blinowski et al, and measured values of the zero-field resistivity. It is shown that, if a reasonable physical model is used for the mobilities, the magnetoresistance cannot be fitted with two- or three-carrier models. Suggestions for the origin of the magnetoresistance are made.

Thermoelectric Power of Graphite Acceptor Compounds

ABSTRACTTemperature variations of the thermopower (TEP) of acceptor graphite intercalation compounds (GIC) are very different from that of pristine graphite. At low temperatures the TEP increases monotonically with T, then levels off above 150 K. This behavior is ascribed to the phonon drag effect. In the region where the TEP is nearly constant, phonon relaxation is mainly controlled by the Rayleigh scattering due to point defects or impurities. This process leads to T-independent phonon drag TEP. The importance of Rayleigh scattering is due to the large cross section diameter of the Fermi surface in GIC. At low temperatures where the boundary scattering becomes important, the TEP is proportional to T3 . Detailed calculations are carried out by solving the phonon-carrier coupled Boltzmann equation.

Low Field Galvanomagnetic Properties of Graphite Acceptor Compounds and their Relation to Trigonal Warping

ABSTRACTIn first stage graphite intercalation compounds with cylindrical Fermi surfaces and isotropic relaxation times, no magnetoresistance occurs. Holzwarth pointed out that trigonal band warping provides a finite magnetoresistance in first stage compounds. A calculation of the galvanomagnetic coefficients of stage-2 acceptor compounds is performed including the effect of trigonal warping on the bands. In the present problem a perturbation calculation provides a good approximation. Without trigonal warping, the stage-2 compounds exhibit a finite magnetoresistance since they have two kinds of holes. Retaining terms up to order (γ3/γ0)2, the weak field magnetoresistance is calculated in limit H→0; Δρ/ρoH2 ≍ (e/h2c)2 (τ/EF)2 (3γ0b/2)4[1/4(γ1/EF)2+57/2(γ3/γ0)2] where b = 1.42 Å is the nearest C-C distance, EF the Fermi energy and τ the carrier relaxation time. The first term in curly brackets is due to the existence of the two kinds of holes. This formula indicates that the term due to trigonal warping is much larger than the other term.

Conduction mechanism in graphite intercalated with certain acceptors

There are a number of problems which must be resolved before it can be said that the mechanism of conduction in acceptor compounds of graphite is understood. In this paper attention will be paid to compounds prepared from AsF 5, SbF 5, and FeCl 3 which exhibit very high conductivities, comparable with that of copper. An analysis of recent data on the temperature dependence of the resistivity and the magnetoresistance will be given and discussed in terms of recent models of the dispersion relationship developed by Blinowski et al. The consequences of screening and the concept of a sandwich model for the conductivity will be examined. Specific values for carrier densities, mobilities, effective masses, Fermi speed, and density of states, will be computed from the models discussed and compared with other data, such as oscillatory phenomena due to magnetic quantization (de Haas-van Alphen, Shubinkov-de Haas, magnetothermal oscillations), magnetic susceptibility, etc. Using these estimates, a discussion will be developed of the effect of crystal defects on the electrical conductivity. Scattering on grain boundaries and effects of crystal misalignment will be considered, since practical synthetic conductors will be based on materials with these defects. The possibility of making inexpensive synthetic conductors will be considered within this context.

Effect of pressure on electronic properties of graphite acceptor compounds

The effect of hydrostatic pressure on the electronic properties of graphite-nitric acid intercalation compounds has been studied. Measurements of the compressibility, the de Haas-van Alphen (dHvA) effect, and the resistivity have been made. The compressibility along the c-axis of the stage 3 compound is (5.0 ± 1.0) × 10 −3 kbar −1, which is about 1.7 times as large as that of pristine graphite. The basal plane compressibility is much smaller in both cases. The pressure coefficients of two prominent dHvA frequencies, 1.9 × 10 6 and 5.8 × 10 5 Oe, of the stage 4 compound are about 1.8 × 10 −2 kbar −1 and nearly zero, respectively. The pressure coefficient of the room temperature c-axis resistivity of the stage 3 compound is 1.4 × 10 −1 kbar −1, about 5 times as large as the corresponding value, 2.9 × 10 −2 kbar −1, of pristine graphite. An anomaly, at several kbar, is found in the pressure dependence of the room temperature resistivity of intercalated samples.

Magnetothermal oscillations, Fermi surface, and band structure of lowest-stage nitric-acid—graphite intercalation compounds

Results on quantum magnetothermal oscillations in graphite acceptor compounds, ${\mathrm{C}}_{8s}$ (HN${\mathrm{O}}_{3}$) $s=2,3,4$ are presented, including de Haas-van Alphen frequencies and effective masses. The frequency spectrum is different for every stage. Combination frequencies are present in every case and are attributed to magnetic breakdown. The interpretation is based on and confirms the independent two-dimensional zone model of the graphite acceptor compounds. A combination of the band-structure calculation of Blinowski and Rigaux and the folding of these bands in the Brillouin zone of the intercalate compound is used. This gives good agreement for the Fermi surfaces and the effective masses. Numerical values are given for the band parameters. Extra parameters are introduced in order to explain the results. For the second stage a charge-transfer ratio of $f=0.6$ and a Fermi-level shift of 1.08 eV are found. For the third stage 99% of the excess charge is localized in the carbon layer adjacent to the intercalate. The electrostatic energy difference between this layer and the inner one is found to be $2\ensuremath{\epsilon}=1.4$ eV.

Intervalence transitions in graphite acceptor compounds

Optical reflectance experiments were performed on several GAC (Br 2, AlCl 3, SbCl 5-graphite) of well-defined stage ( n = 2,3,4). We observed i.r. reflectivity structures whose energies are characteristic of the stage, but depend slightly on the intercalate species. These stage-specific structures are well understood, within the framework of the model of “Graphite Independent Subsystems”, as evidence of the predicted intervalence transitions. With a charge transfer coefficient comparable with the value obtained for the lowest stages of GAC, the expected energies of the intervalence transitions are in good agreement with the experimental positions of the reflectance structures.

Band structure model and electrostatic effects in stages 3 and 4 of graphite acceptor compounds

The electrostatic effects related to nonequivalence of internal and external graphite layers in stages 3 and 4 of graphite acceptor compounds are analyzed in the frame of the model of independent graphite subsystems. The electronic states are calculated by the simplest tight binding method. The band structure is shown to depend crucially on the potential energy difference, 2δ, between external and internal graphite layers. This difference is a function of the excess charge distribution represented in our model by the sum of point charges, dipoles, and quadrupoles localized on atomic sites. The excess charge distribution is related to the electronic wavefunctions, band structure and the position of the Fermi level. The potential energy difference, 2δ, and the fraction of the excess charge localized on internal layers for different total charge transfers has been estimated for the third stage by the approximate self-consistent band structure calculation. The results show that the charge localized on the internal layer represents, typically, about one tenth of the total excess charge. The predominant majority of holes occupies the states in two upper valence bands essentially localized on external layers. The shapes of these bands in the vicinity of the Fermi level are similar in stages 3 and 4, and differ little from the shape of the valence band in 2D graphite.

Some potential applications for intercalation compounds of graphite with high electrical conductivity

Intercalation compounds of graphite of the acceptor type have potential engineering applications because of their attractive electrical conductivity properties. Two kinds of applications are considered in this paper. The first concerns a composite, formed by enclosing an intercalation compound synthesized from high quality crystalline graphite in a matrix of copper. With this form of composite it is found that there are both intrinsic and extrinsic advantages pertaining to the use of a material that has a conductivity higher than and a density lower than that of copper. The second form is a composite compound of intercalated graphite fibers contained in a matrix of epoxy. Extraordinary advantages in this case result from the fact that while intercalation of the fibers produces an order of magnitude increase in their electrical conductivity, when these fibers are incorporated into an epoxy matrix, the composite conductivity is increased by two orders of magnitude over its pristine fiber counterpart. It is projected that these desirable electrical conductivity characteristics portend large scale uses for the acceptor compounds of graphite as substitutes for the present standard conductors, and as a way of upgrading the performance of carbon/graphite materials.

Band structure model and dynamical dielectric function in lowest stages of graphite acceptor compounds

Band structure model and dynamical dielectric function in lowest stages of graphite acceptor compounds

The electronic structure of the graphite acceptor compounds

Abstract Quantum-magnetothermal oscillations in dilute bromine and low stage nitric acid graphite acceptor intercalation compounds are presented. The results are interpreted by using a 2-D electronic structure. For this we assume that localized electrons of the intercalate layer screen out completely each from the other the carbon layers surrounding from each side the interface. These carbon layers are charged with delocalized holes.