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Nanoscopic AFM studies of electrobrightening of stainless steel show that, under well defined experimental conditions, a regular network of hexagonal cells (100 nm wide) can be observed at the surface of this alloy as it was at the electropolished aluminium surface. Its origin is tentatively attributed to an electroconvection process occurring at the metal/electrolyte interface.

Intercalated domains on stage 4 MoCl5–graphite intercalation compounds (MoCl5–GIC’s) were observed by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). On large intercalated domains, a superstructure was found, in relation with a modulation of the electronic properties of the first layer of carbon. From that, the structure of the chloride ions layer was discussed and a model including dimer molecules was proposed. At the boundaries between large intercalated and nonintercalated domains, corrugations were observed along certain crystallographic directions of graphite. Their morphology was studied in detail at atomic scale and formation mechanisms were proposed. Small intercalated domains were also observed. Their shapes were irregular but their boundaries were clear cut.

Current-voltage characteristics, critical current density and thermal conductivity measurements have been performed on YBaCuO/Ag composites. From current-voltage characteristics and critical current density measurements, it is shown that the YBaCuO/Ag composites can be viewed as superconducting link networks. Current-voltage characteristics and critical current density seem to obey a universal law. It has also been demonstrated that the thermal conductivity of the composites is dominated by the electronic contribution of silver.

A single crystal of the two-dimensional structure [NH3(CH2)4NH3]MnCl4 is studied by Electron Paramagnetic Resonance (EPR) when rotated around a ∗ , b and c axes, at X-band and room temperature. Magnetic susceptibility was also measured on a powder sample between 5 and 300 K. Intralayer exchange interaction J k = (4.6 ± 0.4) K calculated from anisotropy of the EPR linewidth is in good agreement with the value |J| k = (4.4 ± 0.1) K deduced from magnetic susceptibility measurements.

It is shown that the onset of the third harmonic susceptibility is the only appropriate criterium, when using AC susceptibility measurements, to determine the IL. Then, we show that the IL strongly depends on the crystallinity of superconductors and on the superconductor volume fraction in the composites. Besides, it is demonstrated, from transport measurements, that the IL corresponds to the J c = 0 line.

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

The thermal stability of fluorine-intercalated carbon fibres has been studied using differential thermal analysis and thermogravimetry. The selected host fibres were polyacrylonitrile-based and pitch-based carbon fibres, either as-received or heat-treated at 3000 °C. Fluorination performed at room temperature in the presence of hydrogen fluoride led to stage-1 compounds for the most graphitized samples. The thermal stability was found to depend on the host structure and on the amount of intercalated fluorine.

The structural and magnetic phase transitions in the series of compounds (CnH2n + 1 NH3)2MnCl4 with n = 5,7, and 9 are studied by means of X-ray diffraction, differential scanning calorimetry, and magnetic susceptibility experiments. Like the previously studied first members of the series (n = 1 to 3), these perovskite-like layer compounds undergo at low temperature an antiferromagnetic transition with weak ferromagnetic moment due to spin canting. In the paramagnetic domain, the magnetic susceptibility is sensitive to a first-order phase transition which could be due to the freezing of alkyl ammonium groups. Les transitions magnetiques et structurales dans la serie de composes (CnH2n + 1 NH3)2MnCl4 avec n = 5, 7, et 9 sont etudiees par diffraction des rayons X, calorimetrie et mesures de susceptibilite magnetique. Comme les premiers membres de la serie precedemment etudies (n = 1 a 3), ces composes lamellaires de type perovskite presentent, a basse temperature, une transition antiferromagnetique avec un ferromagnetisme faible resultant d'un “spin-canting”. Dans le domaine paramagnetique, la susceptibilite est sensible a une transition structurale du premier ordre qui est vraisemblablement due au gel des groupements alkyl-ammonium.

Intercalation of P-120 pitch-based fibers with bromine, from the vapor phase or liquid bromine, has been investigated. The maximum weight uptake was 20–25%, nearly independent of intercalation temperature. Intercalated samples were characterized by X-ray diffraction, Raman scattering and differential scanning calorimetry. Thermogravimetry experiments showed that a large amount (up to 50%) of bromine remains trapped in the fibers up to temperatures in the order of 1000 °C. Measurements of the transport properties (electrical conductivity, magnetoresistance and Hall coefficient) gave estimates of the concentration and mobilities of the charge carriers and of the charge transfer from carbon to bromine (0.5–0.6 electronic charge per Br2).

A high resolution transmission electron microscopy study of fluorine-intercalated carbon fibers has been carried out on two types of host materials: high-temperature treated pitch-based and PAN-based fibers. TEM images and corresponding Fourier transforms have shown that only stage-1 compounds and graphitic-type domains are detected in the experiment conditions. In particular, no interlayer spacing corresponding to higher staged materials have been noticed. The observed results can be correlated to the ratio of semi-ionic to covalent FC bonding previously determined by XPS. The disorder mostly depends on the nature of the host fiber and on the amount of mobile species that may react with the graphene layers during the deintercalation process, which is favored by the high-vacuum conditions.

We have studied the magnetism of annite mica (the Fe-end-member layer silicate having ideal structural formula: {K+}[Fe32+] 10 K and as large as Tc = 58 K in the true end member and a metamagnetic ground state spin structure (predominantly ferromagnetic layers with in-plane moments, stacked antiferromagnetically along the c∗-axis) that is stable despite significant amounts of Fe3+, diamagnetic octahedral ions, and Mn2+ in the natural sample. We propose that the antiferromagnetic stacking is part of the intrinsic zero-field magnetic domain structure and that it is entirely due to dipole-dipole forces. The most striking features are that a paramagnetic Fe2+ temperature dependent fraction persists far below Tc and that the Fe3+ moments abruptly disorder significantly below Tc, with the tetrahedral Fe3+ going first as temperature is increased. Plausible causes for the latter intrinsic features are discussed.

We report experimental data on the resistive transition of granular superconductive composites made of YBaCuO grains sintered in a CuO matrix as a function of temperature and grain concentration. The grain concentration dependence of the sample resistivity at constant temperature may exhibit two percolation transitions: a poor conductor-to-good conductor transition, which is present at every temperature, and a good conductor-to-superconductor transition below the grain superconducting transition temperature. We show that the temperature dependence of the superconductive critical concentration gives informations on the probability function describing the weak superconductive link transition to the normal state.

Characterization of powder samples of Zn 2− x M x P 2 O 7 (M = Mn, Cu; x = 0–2) shows that substitution of small amounts of Cu or Mn into low temperature or α-Zn 2 P 2 O 7 stabilizes a solid form isostructural with β-Zn 2 P 2 O 7 , the high temperature form. Single crystal X-ray analysis of ZnCuP 2 O 7 ; monoclinic space group C 2/ m , a = 6.741(5), b = 8.190(4), c = 4.542(3), β = 107.72(5)°, D calc = 4.210 g cm −3 , Z = 2, R = 6.6% for 353 observed reflections; shows mixed metal sites with 50% occupancy each of Zn and Cu. Octahedrally coordinated metal atoms lie on planes in hexagonal array with two oxygen atoms bridging adjacent metal sites. Magnetic susceptibility measurements of ZnMP 2 O 7 , M = Mn, Cu show antiferromagnetic interactions between cations which are stronger for M = Cu.

The solid solution, Cu 2−x Ni x P 2 O 7 (0≤x≤2), has been synthesized and characterized by X-ray diffraction on powder. A study of the crystal chemistry of this system has revealed the existence of a phase, Cu 2−x Ni x P 2 O 7 (0.04≤x≤1.75), isomorphous with the high temperature forms: β-Cu 2 P 2 O 7 and β-Ni 2 P 2 O 7 . The magnetism of this phase has been measured at x=0.25, 0.5, 1.0, 1.5 and 1.75 between 5.2 and 300 K. Antiferromagnetic interactions between ions weaken with the substitution of Ni 2+ for Cu 2+ in the solid state lattice of Cu 2 P 2 O 7 .

Kapton polyimide and high-modulus polyimide (PPT) films were carbonized and graphitized at various temperatures from 600 to 3000-degrees-C. Their surface was studied by atomic-force microscopy and/or by scanning tunneling microscopy in order to follow the modification of the large-scale morphology and the atomic structure as a function of the heat-treatment temperature (HTT). On the pristine Kapton films, the local order of the molecules is brought to the fore. With increasing HTT (600 to 1000-degrees-C) the structure becomes more disordered while at larger scale a bumpy morphology appears. During graphitization, the bumpy morphology gradually disappears and is replaced by graphitized terraces whose size increases with HTT. At atomic scale, it is shown that the graphene layers progressively grow for HTT higher than 1800-degrees-C. On the films treated between 1800 and 2400-degrees-C, graphene layers containing point defects are imaged and (square-root 3 X square-root 3)R 30-degrees superstructures are observed near large defects. On the samples treated at 2400 and 2600-degrees-C, moire patterns are observed and are attributed to stacking faults (turbostratic structure).

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.

We demonstrate that the skin effect and eddy currents in 123 / Ag composite have a strong influence on the real χ' and imaginary χ parts of the susceptibility measured by the AC technique. We think that the interpretation of susceptibility measurements (even made at low frequency) in terms of the critical-state model for determining critical currents on superconductor/good conductor composites is questionable because of the strong influence of metal addition on the imaginary part of the susceptibility.

Scanning tunneling microscopy (STM) has been used to image at atomic resolution the surface of pyrocarbons deposited at 1680-degrees-C and heat-treated at temperatures up to 2800-degrees-C. Information is obtained on the increase of surface order with HTT. Of special interest are the presence, in HTT 2000 samples, of point defects arranged in a (square-root 7 x square-root 7) R 19. 1-degrees superstructure and attributed to an array of interstitial carbon atoms, and the observation of a moire pattern in the HTT 2200 sample, which could be due to the turbostratic nature of the layer stacking.

Crystalline K 2 MnP 2 O 7 : monoclinic, P2 1 /n, a=12.649(6), b=9.571(3) A β=104.45(3) o , Z=4, λMoK α =0.71069 A, R/R w =4.1/5.4%, 869 obs. refl., shows a packing of PO 4 and MnO 6 polyhedra surrounding tunnels (parallel to the b axis) which contain the K + ions. Mn 2+ ions are found at the comers of planar networks of hexagons parallel to the crystallographic (100) plane. Magnetic measurements on powder (2-300 K) show a maximum χ M at 2.6 K which could be associated with three-dimensional magnetic order

Molecular assemblies were studied at the air-water interface, as were neutral and oxidized Langmuir-Blodgett (LB) films in the mixed (1: n) molecular system of EDTTTF (SC18)2 (EDTTTF, ethylenedithiodioctadecylthiotetrathiafulvalene) and behenic acid (CH3(CH2)20 COOH). At the air-water interface, a possible molecular organization was discussed from the study of the area occupied by EDTTTF (SC18)2 and n molar fraction of behenic acid at a surface pressure of 30 mN m-1. EDTTTF (SC18)2 molecules form dimers and with increasing behenic acid content, a monolayer reorganization occurs. The formation of monomolecular layers including monomer EDTTTF (SC18)2 molecules was achieved for n>or=2. X-ray diffraction studies were concentrated on samples of Y-type LB multilayered films with n=0, 1 and 3. Throughout the mixed (1:n) system, stacking periodicity was dominantly observed from (001) reflections as 55+or-1 AA for neutral films, 72+or-1 AA for a brown-coloured film after exposure to iodine, and 46+or-1 AA for the stable, conducting and purple-coloured film after a thermal activation process. Possible molecular assemblies based upon centrosymmetrical bilayers were qualitatively discussed based on the calculation of the linear electron density profiles of pure and doped EDTTTF (SC18)2 LB films. The large variation of the stacking periodicity during the oxidation process was explained by the molecular inclination of EDTTTF (SC18)2 molecule associated with the insertion of I3- ions which intercalate between EDTTTF (SC18)2 layers.

Mixtures of ethylenedithio-dioctadecylthio-tetrathiafulvalene (EDT-TTF(SC 18 ) 2 ) with behenic acid form Langmuir-Blodgett films of Y-type characterized by islands of EDT-TTF(SC 18 ) 2 in a matrix of the fatty acid. A two-step oxidation (immersion in I 2 vapours followed by heating at 40°C) gives: • - an insulating brown phase with I 3 -isolated species. • - a conductive violet phase with I 3 -chains associated with a mixed valence state and a large structural reorganization.

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.

We have studied magnetic layered materials that have in-plane ferromagnetic exchange interactions, and dipole–dipole only interplane interactions. These are biotite mica (including a nearly ideal annite end member) and new graphite bi-intercalation compounds that contain regularly stacked arrays of ferromagnetic and diamagnetic intercalates. All these strictly (exchangewise) two-dimensional materials exhibit hysteretic spin-glass magnetization cusps at temperatures up to 42 K. This proves that, in the presence of ferromagnetic correlations, classical dipole–dipole forces can play an important role in causing spin-glass behaviour at elevated temperatures. The layered materials described are ideal model systems for the study of a class of spin glasses that require only ferromagnetic exchange interactions. Below some characteristic temperature TFC, the latter interactions induce ferromagnetic correlations that give rise to significant dipole–dipole coupling leading to "domain configuration trapping" and an onset of temperature hysteresis at T ~ TFC.

We have obtained at 1173 K new diphosphates ( M , Cu) 2 P 2 O 7 with M = Mg, Ca, Sr, and Ba. The β-forms of Mg 2 P 2 O 7 and Cu 2 P 2 O 7 appear with a small quantity of Cu 2+ and Mg 2+ ions in their structure, respectively. The solid-solution β -Mg 2− x Cu x P 2 O 7 (0.04 ≤ x ≤ 1.94) at 298 K is continuous and isomorphous to β-Mg 2 P 2 O 7 and β-Cu 2 P 2 O 7 ; its limits were evaluated with DTA and X-ray techniques. Only three new phases CaCuP 2 O 7 , SrCuP 2 O 7 , and BaCuP 2 O 7 were isolated in the ( A , Cu) P 2 O 7 systems, A = Ca, Sr, and Ba. The two former are isomorphous to α-Ca 2 P 2 O 7 , while the X-ray spectrum of the latter is similar to that of BaCoP 2 O 7 and BaNiP 2 O 7 .

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.

The physical properties of the new complex and mixed salt (TMA+)3(I − 3 ) (TCNQ−)2(TCNQo) characterized recently are presented. Crystallographic study at room temperature has shown a diffuse scattering which indicates the existence of a 1-D superlattice. From dynamic (EPR) and static magnetism investigations and the existence of a specific heat anomaly an electronic PEIERLS distorsion is postulated. However the semi conductive behaviour of this salt is evidenced by the d.c. conductivity and the thermoelectric power results. A comparison with similar situations in TTF-TCNQ and mainly in KCP is presented.

In this paper, we discuss an irreversible phase transition occuring in the TCNQ salt which diethylcyclohexylammonium cation : DECA (TCNQ)2, when heated above 350 K. The transition, which is of first order as evidenced by heat capacity measurements, is accompanied by very large discontinuities in properties such as magnetic susceptibility and d. c. electrical conductivity. The phase diagram [P = f(T)] has been determined, as well as the crystal structures of both phases. The observed structural transformation allows us to explain the temperature behaviour of the physical properties.

The phase transitions occurring in the perovskite type two-dimensional molecular composite NH3-(CH2)4-NH3 CuCl4, have been studied by D. S. C., X-ray diffraction, EPR and magnetic susceptibility measurements. This compound exhibits a reversible first-order structural phase transition at T = 325 K, between two monoclinic phases III (P2/c) and II (P21/c). This transition is mainly characterized by an important increase of the interlayer metallic distance (+0.137nm) and a shortening of the intralayer metallic distance Cu-Cu (−0.017 nm). These features are due to a conformational change of the molecular part [NH3-(CH2)4-NH3]2+ from left-handed to all-trans conformation and affect noticeably the magnetic properties of the compound. At higher temperature (T = 423 K), a second-order phase transition leading to an orthorhombic phase I (Pncm) is observed by powder X-ray experiments.

The new charge transfer complex t-TTF-TCNQ, whose unsymmetric cation is intermediate between TTF and HMTTF, presents a regular stacking of both donor and acceptor chains. We show that this compound has a metallic behavior at high temperatures and undergoes one metal-insulator transition near 81 K. Its electrical and magnetic properties are examined in connection to this phase transition and the effective dimensionality.

Intercalation of RuF5 and OsF5 into graphite has been investigated using the vapor phase method. Stage 3 to stage 1 compounds were obtained depending on experimental conditions. The unit-cell constants of the intercalated layers were determined on Weissenberg patterns of a single crystal of C20RuF4.5 composition. A possible arrangement of (RuF4)n chains containing ruthenium in octahedral coordination is proposed. A charge transfer of 0.5 electron per intercalated RuF4.5 unit is deduced from the variation of CC bond length. Structural phase transitions have been investigated on single crystals between 120 K and room temperature. The material shows a metallic behavior and a hysteresis phenomenon occurring around 130 K in the resistivity variation may result from an incommensurate ⇌ commensurate transition. The magnetic behavior of Ru- and Os-intercalated salts is completely different from that of pure pentafluoride and corresponds to Ru(+IV) and Os(+V) species, respectively.

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.

In this paper we report the preparation, crystal structure and some physical properties of the conducting molecular (Perylene) 2 [Pt(S 4 C 4 (CN) 4 )]. The crystal structure consists of segregated stacks of perylene and Pt S 4 C 4 (CN) 4 units, with a spacing of 3.3 A between the carbon atoms in the perylenes. The temperature dependence of the electrical conductivity is metallic with a maximum at approximately 15 K. At this temperature a sharp transition to an insulating state occurs. Magnetic suceptibility measurements confirm the existence of a transition in this temperature range.

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.

On etudie la chaleur specifique de sels doubles de TCNQ a cation diamagnetique du type ammonium quaternaire. Les resultats des mesures effectuees dans certains cas entre 0,2 et 300 K sont compares aux variations thermiques de susceptibilite magnetique. Deux types de complexes sont examines: ceux constitues d'un empilement irregulier des plans de TCNQ et se comportant comme des semi-conducteurs et ceux constitues d'un empilement regulier possedant une conductivite electrique metallique mais a temperature ambiante seulement. Ces derniers presentent un terme lineaire de chaleur specifique et un paramagnetisme non nul aux tres basses temperatures. Pour en rendre compte, on montre qu'il faut appliquer un modele de Hubbard “etendu” unidimensionnel. Une analyse theorique de ce modele ou les interactions electron-electron sont dominantes permet d'interpreter les resultats obtenus. The complex TCNQ salts with quaternary ammonium diamagnetic cations are investigated by specific heat measurements between 0.2 and 300 K and compared with magnetic susceptibility investigations. Two kinds of complexes are examined: those with non-regular stacks of TCNQ entities which are semiconductors and those with regular stacks which are almost metallic around room temperature only. To interpret the linear term of specific heat and the constant paramagnetism observed at low temperatures on this second kind of salts an “extended” Hubbard model for one-dimensional systems is applied. The theoretical analysis based on electron-electron interaction allows to give an explanation of the experimental results.

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.