Molecular motions in the ternary graphite intercalation compounds with potassium and methylbenzenes

Molecular dynamics in the ternary graphite intercalation compounds (GICs) with potassium and methylbenzenes (toluene and o-xylene) have been investigated. It was found that the proton spin-lattice relaxation is determined by two different molecular motions. At low temperatures the relaxation process is monitored by the three-fold methyl group reorientations. At high temperatures the motion is most likely to be the rotation of the phenyl ring around the two-fold axes. The relaxation measurements are discussed in terms of the previously proposed structure model of the ternary GICs with potassium and aromatic hydrocarbons.     

Electrochemical applications of graphite intercalation compounds

Real and potential applications of graphite intercalation compounds in electrochemical processes are critically surveyed. Special attention is given to the fields of “batteries” and “chemically modified carbon electrodes”.Some recent results concerning both the discharge mechanism of graphite oxide positives in organic electrolyte-lithium batteries, and the preparation of metal-doped carbon electrodes via graphite compounds with ion exchange behaviour, are presented in detail.     

Electrochemical synthesis of graphite intercalation compounds with nickel and hydroxides

Graphite intercalation compounds (GIGs) with nickel and iron hydroxides were synthesized from the corresponding GICs with metal chlorides by galvanostatic oxidation and also by repeated charge-discharge cycles in an alkaline secondary battery with KOH aqueous solution. The stage-one structure in the original chloride-GICs was found to disappear after the first charge-discharge process and the compounds kept the stage-two structure. A gradual increase in the capacity of the battery with charge-discharge repetition suggests a slow replacement of chloride ions by hydroxide ones in the graphite interlayer space.     

Electrochemical and chemical reactivity of graphite intercalation compounds with CrO3

The electrochemical oxidation of graphite-CrO₃ intercalation compound (GIC-CrO₃) prepared by the impregnation-dry method allowed lower chromium oxides to be removed, while they are unaffected by chemical treatment in hot 6 N HCl. The Cr(VI) formed from these oxides due to anodization in 0.5 N H₂SO₄ is equal to only 0.11% of the total chromium. The removal of the lower oxides restores the electrochemical properties of the host graphite, which suggests these oxides are bonded to the graphite structure. SEM and electron microprobe analysis showed that the concentration of the intercalated chromium oxides is higher near the flake edges. The behaviour of GIC-CrO₃s prepared by the dry, the impregnation-dry and the solvent methods was then examined in cold and hot solutions of KOH of different concentrations. On boiling with 5 N KOH, the sample prepared by the solvent method appeared to be more readily deintercalated than that of the impregnation-dry method. The reasons for this behaviour are considered in terms of the effect of the intercalation conditions on the structural properties.     

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.     

Thermal analysis of graphite intercalation compounds with FeCl3

The behaviour of graphite intercalation compounds with FeCl₃ (1st and 2nd stages) was characterized by a number of thermal analysis methods up to 800 K. The compounds were shown to undergo reversible solid phase transformations and were subjected to thermal destruction. Temperature, kinetic, and energy characteristics of thermally stimulated processes in the compounds under different ambient conditions are given. An assumption is made about a pyrolysis mechanism for different temperature ranges. A conclusion is drawn about the slightly higher thermal stability of the 2nd stage as compared with the 1st stage graphite intercalation compounds with FeCl₃.     

On the nature of AsF5 in its graphite intercalation compounds

The behavior of fluorinated and unfluorinated graphite intercalation compounds of AsF₅ towards several oxidizing, fluorinating and/or adduct-forming fluorides and fluorine itself, suggests that AsF₅ is intercalated largely in its molecular form. Intercalation compounds of higher stages yield labile ternary intercalates with the adduct forming fluorides N₂F₄, trans-N₂F₂ and SF₄. Iodine oxide pentafluoride forms a ternary AsF₅IOF₅ compound in which iodine retains its oxidation state, +VII. First stage AsF₅ compounds reduce N₂F₄ to trans-N₂F₂ indicating intermediate formation of a labile adduct, N₂F₄·AsF₅, inside the host lattice.     

Magnetic properties of stage-2 random-mixture graphite intercalation compounds and their bulk intercalants

The magnetic properties of stage-2 Co_cM_1−cCl₂ graphite intercalation compounds (GICs) (M = Ni and Mn, 0 ≤ c ≤ 1) and their bulk intercalants, Co_cM_1−cCl₂, have been studied by using d.c. magnetic susceptibility. In stage-2 Co_cM_1−cCl₂ GIC, the adjacent Co_cM_1−cCl₂ layers are separated by two graphite layers. Due to the graphite host effect, demonstrated by decreasing the antiferromagnetic interplanar exchange interaction and island formation in the intercalate layer, the magnetic phase transition of stage-2 Co_cM_1−cCl₂ GIC is very different from that of Co_cM_1−cCl₂. Stage-2 Co_cNi_1−cCl₂ GIC with 0 ≤ c ≤ 1 and stage-2 Co_cMn_1−cCl₂ GIC with 0.45 ≤ c ≤ 1 undergo a two-dimensional ferromagnetic phase transition at the critical temperature T_c, while their bulk intercalants undergo a three-dimensional antiferromagnetic phase transition at the Néel temperature T_N. The magnetic properties of stage-2 GICs are compared with those of their respective bulk intercalants in the light of the graphite host effect. The magnetic properties of stage-2 Co_cMn_1−cCl₂ GIC and Co_cMn_1−cCl₂ are also compared with those of stage-2 Co_cMg_1−cCl₂ GIC and Co_cMg_1−cCl₂.     

EXAFS study of Br2-graphite intercalation compounds

Extended X-ray absorption fine structure (EXAFS) measurements are presented for dilute graphite-Br₂ intercalation compounds containing 0.27 mole % and 0.75 mole % Br₂. Intercalated Br₂ molecules are found to have a BrBr distance of $\text{2.34 ± 0.02}\text{A}\text{?}$ and predominate in the 0.75% sample. A unique feature of the present work is the discovery of another type of bromine molecule with a BrBr distance of $\text{2.53 ± 0.03}\text{A}\text{?}$. This molecule seems to be associated with defect or edge sites and predominates in the 0.27% sample. It is reasonable to speculate that this molecule acts like a “can opener”, preparing the graphite planes for intercalation. The implication of these results with respect to charge transfer is discussed, and it is estimated that roughly 0.16 electrons are transferred to each intercalated molecule, while ～0.6 electrons are transferred to each “can opener” molecule.     

Lamellar graphite compounds with potassium and benzene: structure and transformations

The thermal, oxidation and hydrolytic stabilities of the ternary lamellar graphite compound C₂₄K(C₆H₆)₃ have been investigated. The results of the study have suggested a model C₂₄K(C₆H₆)₃ structure according to which benzene penetrates both the interlayer space containing potassium and the unfilled interlayer graphite space, the benzene molecules being arranged at right angles to the carbon layers of graphite.     

Hydrogen in alkali-metal-graphite intercalation compounds

The sorption and desorption of hydrogen by alkali-metal-graphite intercalation compounds (AGICs), MC₈ and MC₂₄ (M = K, Rb and Cs), were studied by the constant-volume closed method and mass-analyzed thermal desorption spectroscopy. Three forms of hydrogen were detected in KC₈H_x and KC₂₄H_x, while two forms in RbC₈H_x and one form in RbC₂₄H_x were detected. The energy diagrams for the systems composed of AGICs and hydrogen demonstrate that the type of alkali metal intercalated in graphite and the stage structure of AGICs have greater influence on the absorption process than on the adsorption process of hydrogen.     

Alkali-metal-graphite intercalation compounds prepared from flexible graphite sheets exhibiting high air stability and electrical conductivity

The practical use of graphite intercalation compounds (GICs) as industrial materials has been considered to be unfeasible because their air stability is fairly low, notwithstanding their high electrical conductivities and low densities. On the other hand, some air-stable GICs that exhibit p-type conduction have been reported. In this study, air-stable and electrically conductive GICs that exhibited n-type conduction were prepared from commercially available flexible graphite sheets (PGS graphite sheets). We observed that K-GIC and Cs-GIC were air-stable enough to be used in practical applications and were highly electrically conductive. The air stability of Cs-GIC was slightly higher than that of K-GIC. In addition, the stage-2 structure of Cs-GIC was more stable in that it maintained ten times higher electrical conductivity than that of the host graphite after 10 years of exposure to air. Moreover, Cs-ethylene-GIC was the most stable and exhibited a blue colored surface after 10 years of exposure to air. We assumed that the significantly improved air stability of the alkali-metal-GICs prepared from PGS graphite sheets is caused by the large crystalline size and the bending graphite layer structure of the host PGS graphite.     

Resistivity and E.S.R. studies of graphite HOPG/fluorine intercalation compounds

We present conductivity and e.s.r. studies of graphite HOPG/fluorine intercalation compounds. The in-plane resistivity, ϱ_a, was measured as a function of fluorine concentration and temperature using a contactless technique. The c-axis resistivity, ϱ_c, was measured as a function of temperature and hydrostatic pressure using the four-contact technique. The main features of our results can be summarized as follows:

• 1.(1) The conductivity along the planes increases initially with fluorine concentration and reaches a maximum conductivity of σ_a/σ₀ ≅ 11 (σ₀ is the in-plane conductivity of HOPG). Above a concentration of x = 0.18 in C_{1 − x}F_x the conductivity, σ_a, dramatically decreases.
• 2.(2) ϱ_a is ‘metallic’ for concentrations x < 0.2 and can be fitted to the equation: ϱ_a = A + BT + CT². For x = 0.25, the in-plane resistivity is significantly larger and temperature independent.
• 3.(3) The c-axis resistivity, ϱ_c, versus temperature exhibits a clear maximum for x < 0.2 but is anomalously large and almost temperature independent for x = 0.25.

We suggest that carrier localization due to structural deformation and consequent changes in the band structure are responsible for the limiting conductivity. However, a percolation mechanism and domain-wall formation might also play an important role. The maximum in c-axis resistivity is attributed to a cross-over between two mechanisms of conduction: ‘tunneling’ or ‘conducting path’ mechanisms at low temperatures, but a hopping mechanism at high temperatures. The activation energy for hopping is extracted, as well as its pressure dependence. Finally, we provide a critical analysis of previous interpretations of carrier spin resonance in GICs. We have critically checked the theory of Dyson by measuring the A/B ratio, ϱ_a and ϱ_cversus temperatures on the same samples. We demonstrate that extracting bulk resistivities from the e.s.r. lineshape may not be justified.     

Discrete–variational Xα calculations of graphite and alkali–metal graphite intercalation compounds

First-principles molecular orbital calculations using discrete–variational (DV)-Xα method are carried out on the model clusters of graphite and alkali–metal graphite intercalation compounds MC_x(M=Li, Na, K, Rb, and Cs). Results of the calculations are used so as to simulate the experimentally observed near-edge X-ray-absorption fine-structures (XANES) and ultraviolet photoelectron spectra. For the clusters of graphite and KC_x, the calculated partial electronic densities of states (PDOSs) are in good agreement with the experimental C K-edge and K K-edge XANES spectra, respectively. Furthermore, the accordance between the calculated DOSs and the observed UPS spectra of graphite and RbC_x is also satisfactory. It is shown that the Fermi level is pushed up into the conduction band of graphite by doping alkali metals.     

Ferromagnetic resonance study of reduced NiCl2 graphite intercalation compound

A commercial reduction compound of graphite intercalated with NiCl₂ has been investigated by FMR. The X-band measurements for 573 ? T ? 3.5 K and Q-band for 293 ? T ? 150 K show the presence of metallic Ni and some unreduced NiCl₂. The Ni resonance signals have very large linewidths, δH_pp ～ 1800 ? 2000 Oe for 300 ? T ? 150 K. Analysis of the lineshape indicates that Ni remains intercalated in the graphite. The intercalated Ni exhibits a Curie temperature which is approximately 50 K lower than that of bulk Ni. The NiCl₂ X-band signals observed for 150 ? T 3.7 K show a maximum in intensity and linewidth at about 15 K, which is attributed to thermally induced magnetic excitations in the remaining finite NiCl₂ clusters. An in-plane exchange constant, J/k = 15 K, is estimated for NiCl₂ from the temperature dependence of the signal intensity.     

Electrochemical intercalation of HClO4 into graphite and CrO3-graphite intercalation compound

Electrochemical intercalation of HClO₄ into both the graphite flakes and stage-3 graphite intercalation compound with CrO₃ (CrO₃GIC) is discussed on the basis of the slow potentiodynamic scanning and X-ray diffraction analysis. It has been shown that intercalation of HClO₄ is much faster into CrO₃GIC than into the pristine graphite. The differences in the intercalation/deintercalation processes depending on the kind of host matrix indicated that the ternary stage-1 CrO₃HClO₄GIC (TGIC) is formed due to the insertion of the HClO₄ intercalate into the host CrO₃GIC. In contrast to the binary HClO₄GIC, the cathodic deintercalation of stage-1 CrO₃HClO₄TGIC and stage-1 TGIC-graphite oxide systems leads to the restoration of the starting stage-3 structure instead of pure graphite. As compared to HClO₄GIC, the cyclic overoxidation of stage-1 CrO₃HClO₄TGIC, resulting in the formation of graphite oxide, needs much less positive potentials to occur. The results of this work give credence to the assumption that bi- and cointercalation phases coexist in the product of the subsequent intercalation of HClO₄ into CrO₃GIC.