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We report here the first observations ai the atomic scale of a symmetrical tilt grain sub-boundary with a STM. Trie edge dislocations observed ai the atomic scale along trie boundary fine can be understood in the trame of geometrical models. These STM images also reveal a local modification of the charge density near the Fermi level in trie junction area. Grain boundaries and grain sub-boundaries are crystallographic defects charactenstic of zone transitions between two crystals with dilferent orientations. The manifestations of this kind of defects at the "macroscopic scale" were the object of numerous studies (using TEM essentially). Other techniques such as High-Resolution Electron Microscopy (HREM) Iii, Grazing-Incidence X-ray Scattering technique (GIXS) (2) or Fresnel contrast analysis in low-resolution images (3) provide, in some cases, information at a smaller scale. However, these techniques are heavy to use and Iimited in their spatial resolution (lateral as well as vertical). Owing to trie development of Scanning Tunneling Microscope (STM) defects such as grain boundaries can be studied at the atomic scale. The lateral resolution achieved with this type of microscope (< 1 A) allows one to study the very fine structure of these defects (atomic displacements in the junction area). This is also of particular interest when the interatomic distances of the studied crystal are less than the lateral resolution of a HREM for example (2 A), in the case of graphite for instance (in-plane C-C distance of1.42 A). Although graphite is one of the most STM studied compound, few observations of grain boundaries were reported on Higuly Oriented Pyrolytic Graphite (HOPG). These stems mainly

Samples of stage-2 fluorine graphite intercalation compounds (GIC) were studied by scanning tunneling microscope (STM) from submicrometer to atomic scale. From the STM view point, the most striking observation concerns the topographic modifications of the ( a , b ) sample surface induced by the intercalation process. Our STM images of a C 6 F compound are the first direct visualization of the fluorine atoms organization in the van der Waals gap.

Fluorine-intercalated graphite fibers have been synthesized at room temperature by direct reaction with elemental fluorine. The host materials were either vapor-grown fibers or pitch-based fibers. The CxF compounds obtained were mostly of the stage-3 type with a 12.65 A repeat distance. The carbon-to-fluorine atomic ratio ranged from 10 to 14. Resistivity measurements carried out between 4.2 and 300 K show clearly that the fluorination reduces the resistivity values, whatever the type of pristine material. For fluorine-intercalated pitch-based fibers an anomalous increase of resistivity below 30 K may be consistent with a weak electronic localization or with two-dimensional carrier-carrier interaction effects. In fluorine-intercalated vapor-grown fibers the anomalous bump observed around 190 K could result from a phase transition. The room temperature value of the conductivity of this type of material, i.e. σ = 1.6 × 105 S cm−1, is one of the highest ever observed for fluorine-intercalated CxF fibers.

An atomic-scale multilayer with two types of ferromagnetic layers is achieved by graphite intercalation. It is a bi-intercalation compound with stacking sequence CrCl3NiCl2NiCl2/… where/represents a graphite layer. The two types of chloride layers are much as they occur in the pristine chlorides and have the same lattice parameters and intralayer intertransition–metal ferromagnetic exchange interactions. The transition temperatures for singly intercalated CrCl3 and NiCl2 graphite compounds are, respectively, 11.3, and 20.2 K. The bi-intercalation compound shows a "double transition" with Tc1 and Tc2 being equal to the relevant intercalate-specific temperatures. It is proposed that, as the temperature is lowered, the NiCl2 layers order first at Tc2 into a 3-D antiferromagnetic stacking of ferromagnetic planes, followed by onset of intra-CrCl3-layer ferromagnetic order at Tc1. Below Tc1 the two types of ferromagnetic planes probably assume a 3-D antiferromagnetic stacking that involve both types of layers, requiring that the NiCl2 stacking be different than at Tc1 c2. The main interlayer interactions are believed to be dipole–dipole forces and these are seen to give large low-field temperature hysteresis effects. This is in contrast to many layered materials with antiferromagnetic in-plane interactions in which: (i) dipole–dipole forces do not play a significant role, (ii) 3-D order is not intercalate specific but occurs at temperatures that are much lower than the in-plane interaction strengths and that are highly dependent on the interlayer interaction strengths, and (iii) hysteresis effects are not observed.

The X-ray absorption spectroscopy (EXAFS) technique has been used for studying the local environment of the metal in graphite intercalation compounds with transition metal dichlorides. For these compounds the octahedral environment of the cation is preserved after the intercalation process. A simulation of the first coordination shell with a two metal-ligand distance distribution enables the characterization of a strong distortion of the octahedra. The study of the radial distribution function around the metal atoms gives evidence of the existence of a second coordination shell.

Static magnetic susceptibility anisotropy ( Δ X and EPR spectroscopy experiments have been carried out in the range 4 – 300 K on high-stage potassium intercalated graphite (HOPG). The magnetic anisotropy measurements show clearly the occurrence of first order phase transitions at about 230 K for KC 72 and 245 K for KC 144 and KC 168 . These transitions are attributable to order-disorder processes inside the potassium layers.

The intercalation of nickel chloride into graphite in presence of chlorine gas has been studied in the temperature range 390 – 750 °C with an apparatus allowing the measurement of the chlorine pressure during intercalation. The results indicate that chlorine takes part in the reaction with a 600 mmHg threshold pressure below which no intercalation occurs. Between 495 and 690 °C, a second stage compound C 11.3 NiCl 2.13 , is obtained with the apparent activation energy of the intercalation being about 50 kcal/mole. X-ray investigations show that intercalated nickel chloride has a structure very close to the nickel chloride sandwich in free metal halide. However, intercalant layers are not complete, with the filling coefficient being at most 0.71. All of the data are consistent with an islandic nature of nickel chloride: islands of diameter 100 A allow explanation of the non integer stoichiometric coefficients given by chemical analyses and the amount of charge transferred from graphite to nickel, which is, moreover, consistent with a decrease of CC bond lengths as observed by X-ray diffraction.

The synthesis of graphite intercalation compounds with transition metal fluorides has been investigated. It is shown that, contrary to other 3d metal fluorides, TiF 4 can be easily intercalated as second-stage compounds, in presence of fluorine gas. From X-ray diffraction studies, evidence is given for the presence in the layers ionic TiF 6 -- and neural TiF 4 species. Finally, the intercalation of metal chlorides and fluorides is compared.

The study of the intercalation of 3d, 4d and 5d fluorides into graphite has been investigated. Novel graphite salts have been synthesized from various types of graphite with an excess of gaseous metal fluoride, either under vacuum or under fluorine atmosphere. Single-crystal structure studies at room temperature allow us to identify a second-stage compound with titanium tetrafluoride ( I C ≊ 11.35 A ) and graphite intercalation salts of different stages with ruthenium and osmium pentafluorides : the first-stage ruthenium compound has the identity period of 8.42 A. The magnetic behavior of the Ru-intercalated salt is completely different from that of pure RuF 5 : it suggested the presence of this element in the (IV) and (III) oxidation states. Conductivity measurements performed on a high-stage sample show a metallic behavior : the strong variation of resistivity observed at about 120 K could be due to an order-disorder transition within the a-b planes.

We have measured the static susceptibility of nominal stage-2 ${\mathrm{CuCl}}_{2}$-graphite intercalation compounds prepared from four different host graphites (highly oriented pyrolitic graphite, natural graphite crystals, and graphite fibers heat treated at 2500 and 3290 \ifmmode^\circ\else\textdegree\fi{}C). The susceptibility and its anisotropy both contain a 1/T component which is shown to be due to finite-size effects and which, when removed from the data, reveals a bulklike behavior that closely corresponds to the pristine behavior for Tg50 K. The behavior at lower temperatures is more complicated but is entirely consistent with an intercalate-island diameter of ${d}_{\mathrm{isl}\mathrm{\ensuremath{\simeq}}}$100 A\r{}, leading to a distribution of finite isolated quasi-Heisenberg antiferromagnetic chains of S=1/2 ${\mathrm{Cu}}^{2+}$ ions.

The intercalation of rhenium chloride into graphite from Re Cl5 in presence of chlorine gas has been studied. The maximum amount of intercaled chloride was obtained around 370°C and corresponded to the formula C9 Re Cl4 which was mostly of first stage with some admixture of higher stages. The crystal structure of the intercalated chloride is hexagonal with a-parameter of 6.50 A. The lattice is rotated by 20° from the carbon lattice: the two lattices are commensurate. Magnetic measurements seemed to show the presence of Re Cl4, although the existence of a mixture of Re Cl5 and Re Cl3 could not be excluded. Finally the electrochemical behaviour showed that the compound could be used as anode material in alkaline batteries.

The effect of temperature, moisture and current density on the stability of pitch-based fibers intercalated with metal chlorides is investigated. Particularly interesting are the high current densities available without degradation: current densities as high as 400 A/mm 2 can be maintained during several weeks without deintercalation. The results are explained using a free convection or radiation model. It is shown that the limiting value of current density is proportional to (ϱ d ) −1 2 under vacuum and to (ϱ d 2 ) −1 2 in gaseous atmosphere, where ϱ and d are the resistivity and fiber diameter, respectively.

Intercalation of graphite with metal chlorides leads to compounds for which chemical analyses give non-integer stoichiometric coefficients. In particular, an excess of chlorine is observed with respect to free metal chlorides. For NiCl 2 intercalated graphite, the composition of the second-stage compound corresponds to the formula: C 11.3 NiCl 2.13 . This chlorine excess was accounted for by assuming the existence of small islands of intercalated chloride. As NiCl 2 -graphite compounds are known to undergo a magnetic phase transition at about 20K, neutron scattering and diffraction experiments were carried out in order to provide evidence for the islands and to specify the magnetic ordering at low temperature. From the small-angle scattering data collected below and above the magnetic transition, the first experimental evidence for the presence of intercalated islands has been obtained. The mean size of the islands is about 150 A, as previously assumed. From diffraction experiments, it has been shown that the spins of Ni 2+ ions in the magnetic phase are aligned ferromagnetically within the intercalant layer.

Because of the poor electrical conductivity of nickel hydroxide, NiCd alkaline batteries have a cathode consisting of a mixture of graphite and nickel hydroxide. The replacement of this two-phase mixture by a single phase consisting of the active material intercalated into graphite, should improve the battery efficiency. To this end, electrodes containing nickel chloride-graphite intercalation compounds underwent successive charge-discharge cycles at various rates. Their electrochemical behavior were comparable with those of classical graphite-nickel hydroxide mixtures. The electrode reactions have been followed by in situ magnetic measurements: during the early cycles, intercalated nickel chloride is converted progressively into nickel hydroxide by replacement of Cl− ions with OH− ions from the electrolyte.

The potentialities of graphite intercalation compounds as electrode materials in batteries are investigated. Emphasis is given to intercalated metal chlorides and their use in aqueous batteries. We show how classical systems can be improved and give examples of possible new electrode materials.

Single crystal structure studies at room temperature have been made for the first stage Mn Cl 2 intercalated graphite. The nominal composition was C 5·6 Mn Cl 2·4 , as deduced from chemical analyses and X-ray diffraction intensities. The data are in agreement with the island model, already proposed for Ni Cl 2 intercalation. From the decrease in C-C bond length and comparison with As F 5 compound, it is shown that the charge transfer is determined by chlorine in excess with respect to free metal halide.

The magnetic properties of graphite-MnCl2 intercalation compounds (stages 1 and 2) were investigated from 3 to 300K by EPR and static susceptibility experiments. The behaviour of the magnetic susceptibility anisotropy, EPR linewidth and g-factor gives evidence for short range order effects increasing sharply below 50K. The data are analyzed in connection with the two-dimensionality of the compounds.

The temperature dependences of g -factor and linewidth of C 11,3 NiCl 2,13 single crystals have been measured from 10 to 400 K. The data indicate that nickel ions in graphite behave at low temperatures like Ising ions, with a phase transition occurring at about 30 K.

Graphite intercalation compounds and doped polyacetylenes are synthetic metals which exhibit many similarities in spite of their different dimensionality (2-d for the former and 1-d for the latter, respectively). After a brief survey of the preparation, structure and properties of polyacetylenes and doped polyacetylenes, a comparison will be made with graphite intercalation compounds. A number of similarities will be shown, concerning, for example, the nature of dopant species, the mechanism of doping, the charge transfer between host and dopant, and even some features of crystal structure.

Conjugated polymers suoh as polyaoetylene (CH)x polyphenylene (C6H4)x' poly thiophene (C4H2S)x' etc. , which are insulators in their pristine state, can be brought to the metallic state after "doping" with ohemioal speoies whioh oan be either eleotron donors or I aoceptors. . This doping prooess involves a oharge transfer between the dopant moleoule and the polymer ohain whioh are then supposed to be spatially olose to each other. It follows that the meohanism of doping must be oonsidered as an aotual interoalation process, which will greatly affeot the struotural oharacteristios of the starting material, as well as its morphology, as has been observed during the 2 intercalation of graphite and layered compounds . In parallel with these modifioations, the band struoture of the system changes yielding a new set of eleotronio properties. It is evident therefore that the struotural and eleotronio properties are intimately related, and must be studied simultaneously in the same system to give reliable information. A great number of studies have been devoted to the structural and electronic properties of conjugated polymers after a chemical or 2 electrochemical doping process . Most of these concern the properties of the system for a given dopant concentration. With this approach a universal pioture of the polymer/dopant system is very diffioult to obtain, as a comparison between different experiments is very hazardous. On the other hand, only a small number of measurements have been performed during the continuous electroohemioal doping of various polymers.

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.