Interlayer and intralayer ordering in SbCl5-GIC's are probed by X-ray diffuse scattering. A crossover from 3-d to quasi 2-d structure is observed at around stage 2. Surprisingly, different in-plane arrangements can be obtained under nominally the same preparation conditions at the same stage. A stripe-like incommensurate structure close to the √39 superperiod of graphite, and a 14×14 commensurate superlattice structure, have been observed in different samples. The formation of these groundstate structures, and their transitions to ‘disordered’ phases, are discussed in terms of the molecular degrees of freedom.

An experimental study is reported of the commensurate (√7 × √7)R19.1° to glass phase transition in stages I < n > 4 graphite-SbCl5 using electron and x-ray diffraction. The electron diffraction studies show an in-plane phase transition to a glass at low temperatures. The (00l) x-ray diffraction scans show that the c-axis layered structure of the intercalate is unchanged by the phase transition. A driving mechanism for the commensurate to glass transition is suggested.

Reported here is the first observation of an incommensurate-commensurate phase transition at 316 ± 1 K upon heating stage-2 graphite-bromine prepared in bromine liquid. The transition during heating involved the movement of the 300 reflection toward a lower q value and the sharpening of the 010 reflection, thereby resulting in the commensurate structure characteristic of stage-2 prepared in bromine vapor. The transition was reversible. The previously reported comtmensurate-incommensurate transition occurred at 326, 329, 332, 338 and 348 K for stage 2 (in bromine liquid), stage 2 (in bromine vapor), stage 3, stage 4, and desorbed material, respectively. Disordering was observed upon completion of this transition.

We summarize the results of an extensive x-ray scattering study of the phase transitions in intercalated graphite-Br2 compounds. The experiments are aimed at understanding the intraplanar and interplanar correlations of the intercalate bromine as a function of temperature. Using in-situ high resolution x-ray scattering techniques, we have studied stage-4 graphite-Br2 under equilibrium conditions, where the intercalant is in the commensurate phase, incommensurate phase, and fluid phase. We demonstrate that the transitions between these phases are model examples of phase transitions in quasi two-dimensional systems. The coherently ordered in-plane bromine regions exceed 10000 Å in size in both commensurate and incommensurate phases. A commensurateincommensurate transition in the 7–fold direction is observed from a centered (√3 × 7) phase to a stripe domain phase. The domain wall density exhibits a power-law behavior in temperature with an exponent of 0.50 ± 0.02. We have also observed power-law lineshapes for bromine superlattice peaks in the incommensurate phase due to the rigorous absence of true long range order. The intercalate layer appears to exhibit an anisotropic melting transition from an incommensurate solid to a novel fluid phase.

In α-C5n-HNO3 the condensation of the room-temperature liquid-like diffuse ring associated with the disorder-order transition around 250 K is studied and the low-temperature. superstructure is examined.

It is found that β-C8n-HNO3 exhibits an in-plane incommensurate order at room temperature.

Two types of graphite-Br2 are found. Low-temperature phase transitions in C8Br are observed at T1 ≍ 277 K and T2 ≍ 297 K. The room-temperature structure of C14Br is reexamined. Special attention is given to diffuse scattering and incommensurability.

High resolution electron imaging supported by computer image simulation has been carried out for a nominal “second–stage” NiCl2 graphite compound. The resulting local structure information underlines the statistical nature of the stage ordering, rarely perfectly regular. It also shows the interpenetration of differently staged regions.

At high pressure KC8 undergoes a staging transition to a mixture of stages 1 and 3/2 at 15 kbar and stages 1 and 2 at 19 kbar. The width of the new peaks are not resolution limited but show a pronounced and non-monotonic variation with Q. This broadening is the result of a finite concentration of staging defects which can be simply modeled by stochastic disordering of an appropriately chosen two component layer sequence. This same analysis has also been used to explain deviations from the Bragg locations in KC84.

Highly graphitized pitch based carbon fibers with heat treatment temperatures (HTT) of 2500°C, and 3000°C have been intercalated with SbCl5, ICl, HNO3, and Br2. These were reacted either by themselves alone, or in double sequence (e.g. Br2 followed by SbCl5). The electrical resistivities of individual fibers were then measured in air, and as a function of temperature, to a series of temperatures, to test air and thermal stability. The lowest resistivity found in this sequence so far is 28 micro ohm cm when ICl intercalation was followed by HNO3 reaction. These conductors are sometimes air stable (depending on intercalant) at room temperature and can be heated somewhat, but begin to deintercalate at much lower temperatures than was found for CuCl2 intercalated fibers [1,2]. In comparison with the CuCl2 reaction with pitch fibers, these intercalants react easily, and thesequential intercalations appear to give lower final resistivities.

The intercalation of various acceptors and donors into graphite fibers, prepared from benzene-derived precursor materials is investigated by Raman spectroscopy, x-ray diffraction, electron diffraction, lattice fringing, and electrical resistivity measurements. Evidence for formation of well-staged acceptor compounds is provided by Debye-Scherrer x-ray diffraction which probes the bulk fiber and by Raman spectroscopy which probes an optical skin depth (< 0.1 μm). Lattice fringing measurements provide direct observation of large regions (up to 50 Aring; × 400 Aring;) of defectfree single-staged regions. Values for the c-axis repeat distance Ic are obtained by indexing (00l) lines of the x-ray diffraction pattern. Raman results show characteristic upshifted modes for stage 1 acceptor compounds with a sharpening in linewidth as compared to the E2g2 mode of the pristine fiber. The room temperature electrical conductivity is increased about an order of magnitude upon intercalation and exhibits a metallic dependence on temperature. The highest air-stable room temperature conductivity 1.4 × 105 (Ω-cm)−l ever reported for an intercalated fiber has been achieved.

Diffraction patterns of carbon fibers have been recorded photographically using monochromatized MoKα radiation. <001> and <hk0> reflections are observed for fibers pyrolyzed at lower temperatures. The absence of <hkl> reflections suggests that the graphite planes are turbostratic. A three-dimensional structural transition is manifest in the appearance of <hkl> reflections. The results suggest that the degree of three dimensionality increases progressively through the sample as a function of increasing temperature of pyrolysis. There is no evidence for coexistence of three dimensional and tubostratic phases. It is further demonstrated that turbostratic carbon fibers may be intercalated to stage two with AsF5.

Individual graphite fibers (TP 4104B and GY-70) were intercalated with alkali metals using the two-zone vapor transport method commonly used to prepare alkali metal intercalated graphite. The progress of the reaction was followed in situ by measuring the electrical resistances of the fibers as the temperature difference (ΔT) between the fiber and the metal was decreased stepwise. These measurements showed that the ease and extent of intercalation are related to fiber graphitization. Without exposure to air, the temperature dependence of the resistances of the intercalated fibers were also measured from −196°C to 400°C. The measurements showed that the intercalated fibers have a metallic dependence on temperature. Tensile strength measurements on the intercalated fibers showed that intercalation of the heavy alkali metals is deleterious.

Carbon 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.

Reflectance spectra of the stage 1 and 2 compounds of graphite intercalated with Hg-K amalgam are reported for the range (0.2-5.8eV). A Kramers-Kronig analysis was performed on the spectra to determine the dielectric function ∊1(ω) and ∊2(ω). The intraband and interband contributions to ∊(ω) have been separated to determine the plasma frequency and free carrier lifetime.

We report optical reflectance measurements on stage 1-4 graphite-SbCl5 intercalation compounds in the energy range 0.2 to 10 eV. The data are analyzed using a Kramers-Kroniq relation to determine the parameters describing the intraband and interband contributions to the dielectric functions ∊1(ω) and ∊2(ω). Structure in these functions are compared with recent band calculations by Blinowski et al.

Reflectivity measurements on graphite-CoCl2 intercalation compounds between 0.06 eV and 2.0 eV for stages 3 and 5 show sharp Drude edges and superimposed interband structure. Stage 5 shows structure near 0.4 eV and 0.55 eV while stage 5 shows structure between 0.12 eV and 0.55 eV. The observed number of interband transitions is equal to the stage index which is consistent with a recently proposed kz-axis zone-folding model of the electronic energy bands. In addition to the electronic optical transitions, features associated with lattice vibrations are observed in the spectra. In particular, the graphite-like IR mode near 1580 cm–1 is clearly resolved in the intercalation compounds. A reflectivity minimum and splitting of this vibrational mode is observed for both stages 3 and 5 of graphite-CoCl2.

Results of Optical Reflectance measurements on stage 1-6 graphite-FeCl3 intercalation compounds are reported. The frequency dependences of the dielectric constants(∊1, ∊2) are determined. The contributions from free carrier and interband absorption are separated to determine important material parameters.

In this paper we present a brief survey of previous work as well as some new preliminary results for graphite fluoride compounds.

First–principle SCF–FLAPW calculations prove the existence in graphite of a new kind of electron states. They are interlayer states, resulting from the bonding combination of states bound to single graphite layers, and give rise to a still undetected energy band just above the Fermi energy. This band also occurs at slightly lower energy in Li intercalated graphite, and has been incorrectly interpreted as arising mainly from Li 2s orbitals.

We employ density functional theory to study structural energies in pure graphite, Li-graphite, and K-graphite. Qualitative agreement with experiment is obtained for the carbon plane binding energy, lattice constants, compressibilities, and in-plane alkali-alkali potentials.

Results of de Haas van Alphen effect study on second-stage bromine graphite intercalation compound C14 Br are presented together with a two dimensional model of electronic structure and Fermi surface. The results are characterized by a mixing of de Haas-van Alphen frequencies. The nature of the coupling mechanisms between orbits is discussed. It is shown that the two dimensional Fermi surface of C14 Br contains nested parallel parts which can stabilize charge density waves. A comparison between the nesting vectors of this Fermi surface and the satellites and diffuse streaks found in structural studies shows a fair agreement between both.