Host Graphite Research Articles

The Low-Temperature Three-Dimensional Structures of the Second-Stage Caesium Graphitide

The three-dimensional (3D) structure of the second-stage CSC24 graphitide was investigated by recording hk.l monochromatic rotating-crystal diffractograms. At low temperature, the two-dimensional (2D) structure is formed of an incommensurate (2.53 x 2.53 R 14.5°) and a commensurate (2 x 2 R 0°) lattice. Below 140 K, whereas the Cs incommensurate layers become 3D ordered, the 2 x 2 R 0° commensurate layers are randomly distributed in the structure. The commensurate structure, with composition CsC16 (stage two), probably equilibrates the lack of Cs induced by the incommensurate structure (composition CSC26), in order to reach the global composition CSC24 of the compound. Depending on the temperature, two kinds of stackings appear. Below 140 K, the incommensurate Cs layers are arranged in a 2D-modulated initial stacking, transformed at a lower temperature into a rhombohedral stacking in which Cs atoms are modulated along the c axis. Simulation of the selection rules allowed the determination of models of the incommensurate layers stacking along the c axis, with translations from one Cs layer to another layer commensurate with the graphite lattice. The initial stacking is described by a pseudo-two-layer model with successive random translations and a short-range order. The structure appearing at a lower temperature is described by a pseudorhombohedral stacking with definite translations. The transition from the initial stacking with a first-neighbour interaction to the pseudorhombohedral stacking with a second-neighbour interaction could be a result of the change in interactions between the Cs atoms and the graphitic host, due to the dominating influence of the graphite periodic potential at low temperature.

Ab initio study of structural properties of stage‐1 alkali graphite intercalation compounds

AbstractFirst‐principles electronic structure calculations are carried out for the stage‐1 alkali graphite intercalation compounds LiC6 and XC8 (X = Li, Na, K, Rb and Cs). Lattice constants and atomic positions are determined by total energy minimization. It is demonstrated that the experimentally observed AA stacking order of the graphite host is partly a consequence of the electronic charge transfer from the alkali atoms to the graphite host.

Physical properties of CVD-grown Se—carbon films

A novel chemical vapor deposition (CVD) approach based on the thermal decomposition of an aromatic hydrocarbon in the presence of Se vapor is used to grow films of layered Se—carbon compounds. In principle, this technique can produce a wide variety of new carbon-based materials, for example, graphite intercalation compounds (GICs) which, for kinetic reasons, cannot be made if the intercalant vapor has to diffuse large distances into a pre-existing graphitic host. In particular, homogeneous oriented submicron films of either pure stage-3 (Se 24C) or mixed-stage Se—carbon layer compounds have been successfully grown on Ni substrates in evacuated sealed quartz tubes. X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering and the c-axis electrical transport measurements are discussed in terms of both covalent Se—carbon bonding and an ionic model assuming the formation of an acceptor-type Se—GIC with electron transfer from carbon to Se. Our CVD-grown Se—carbon films exhibit the largest thermoelectric power reported in the open literature among carbon-based compounds. However, the value is at least a factor of 10 less than reported for these materials in patents by Sharp Corp.

Charge density and charge transfer in stage-1 alkali-graphite intercalation compounds

First-principles electronic structure calculations are carried out for the stage-1 alkali graphite intercalation compounds ${\mathrm{LiC}}_{6}$ and ${\mathrm{XC}}_{8}$ (X=Li, Na, K, Rb, and Cs). We analyze the charge densities and the differences to the reference charge densities of graphite host and intercalant sublattice. For the alkali metals Na, K, Rb, and Cs the computed charge transfer is nearly constant at a value of 0.7 elementary charges (e); values of 0.5e and 0.4e are found for the Li compounds ${\mathrm{LiC}}_{6}$ and ${\mathrm{LiC}}_{8}$, respectively. It is shown that the main fraction, about 0.4e, of the charge transfer is a geometrical consequence of the simple overlap of the charge densities of the graphite and intercalant sublattices.

Hydroformylation of Ethylene via Spontaneous Cell Reactions in the Gas Phase

A new method for the hydroformylation of ethylene using a gas cell composed of (C2H4, CO, cathode |H3PO4aq.| anode (Pt-black), H2) has been examined at ca. 373 K and atmospheric pressure. Among many cathode electrocatalysts tested, H2PtCl6showed the highest selectivity and high activity for the formation of C2H5CHO. However, H2PtCl6did not catalyze the reaction at all under a gas mixture of C2H4, CO, and H2. H2PtCl6was specifically active for the electrocatalytic hydroformylation by separating H2from C2H4and CO with a H3PO4-electrolyte membrane. The addition of Na3PO4to the cathode enhanced the rate of C2H5CHO formation as well as the selectivity. The cell reaction does not require electrical energy input but takes place spontaneously and most efficiently under short-circuit conditions. The results from kinetic and electrochemical studies have suggested that the catalytic active species is the Pt2+-chloride supported on the host graphite. A stronger coordination of CO compared to those of C2H4and hydride reduces both the hydrogenation and hydroformylation of C2H4. Therefore, a low partial pressure of CO and higher partial pressures of C2H4and H2are recommended for the conversion of C2H4. The optimum temperature for the hydroformylation was 373 K. The results of transition response experiments have suggested a rapid reversible coordination of C2H4but a slow conversion of the precursor of C2H5CHO on the Pt2+sites. A tentative reaction mechanism assuming the formations of ethyl–carbonyl, ethyl–carbonyl hydride and acyl–carbonyl Pt complexes has been proposed on the bases of the results in this work.

Determination of the low temperature 2D and 3D structures of the second stage cesium graphitide

Abstract The structure of stage-2 cesium derivatives CsC 24 was investigated using X-ray diffraction. The 2D and 3D arrangements of the cesium atoms in pure CsC 24 were deduced from Laue diffractograms and rotating crystal patterns. Below 165 K, the 2D structure is formed mainly of a 2.54 × 2.54 R 14.5 ° incommensurate structure with 2 × 2 R 0 ° commensurate domains. At 140 K, while the cesium incommensurate layers get 3D stacked, the 2 × 2 R 0 ° commensurate layers are randomly distributed in the structure. The incommensurate cesium layers are arranged in a 2D modulated stacking, but at lower temperatures this structure is partially replaced by a rhombohedral stacking in which the cesium atoms are modulated along the c -axis owing to the dominating influence of graphitic host. In slightly oxidized compounds, the presence of impurities induces the formation of additional 2.23 × 2.23 R 25.5 ° incommensurate domains which are separated from the structures already observed in pure compounds. These stage-2 domains of composition CsC 20 are characterized by a large interlayer distance. We suggest that oxygen suboxide impurities at the edge of the domains are at the origin of this particular dense structure.

Electrochemical intercalation and oxidation of glyoxylic acid into a highly oriented pyrolytic graphite (HOPG) electrode

Abstract The intercalation and electrooxidation of glyoxylic acid in 0.1 M KOH solutions in a highly oriented pyrolytic graphite electrode (HOPGE) have been studied. The values of Tafel slopes, reaction orders and apparent activation enthalpy determined resulted in an experimental rate equation as follows: I = nFK C Glyox exp 0.44 FE RT . A five step mechanism can be given to explain the expression, where a first intercalation step is followed by a single electron transfer from the inserted glyoxylate ions towards the host graphite layers. At the end, CO2, CO and water are obtained as reaction products which give rise to localized strains with the result of a sudden exfoliation of the electrode.

P-T-X conditions of late Hercynian fluid penetration and the origin of granite-hosted gold quartz veins in northwestern Iberia: A multidisciplinary study of fluid inclusions and their chemistry

Quartz veins hosted by late Hercynian granites and their host rocks occur across the Variscides of the northern Iberian peninsula, and locally display Au-As mineralization. Four separate occurrences at Corcoesto, Tomino, Penedono, and Pino have been investigated to determine P-T-X conditions of formation and likely fluid sources. Special attention has been paid to fluid chemistry using a multidisciplinary investigation of fluid inclusion gases by Raman spectroscopy (individual fluid inclusion analysis) and mass spectrometry (bulk analysis), and ion chemistry using microthermometry and bulk leachate chemical analysis. Two major changes in the chemical and physical environment have been identified: (1) a progressive change in the bulk chemical composition from early CO 2-rich, C-H-O-(N) fluids, equilibrated with graphitic metamorphic host rocks, to late H 2O-dominated fluids, inferred from their halogen signature to result from an influx of meteoric or upper crustal fluids affecting the basement at the end of Variscan orogenesis, and (2) changes in the P-T conditions from early stage sulphide deposition in quartz veins, at ca. 450 °C and 150-300 MPa, towards epithermal conditions, ca. 260–310 °C and <75 MPa, at the stage of gold mineralization. Several chemical trends are shown by the fluid inclusions: (1) dilution of the early volatile-rich fluids, (2) a break of graphite buffering activity demonstrated by the CH 4 content increase in the volatile fraction of the latest As mineralizing fluids, and (3) increasing contribution of a relatively oxidizing fluid enriched in sulphate and bromide during the latest stages of fluid percolation (Au stage). These latest fluid stages are interpreted as indicative of extended fluid penetration downward in the crust enhanced by late brittle deformation and decompression, and played an important role in mass transfer at the end of the Hercynian orogeny, especially in transport of metals. Gold ores have formed mostly in granites because main fluid pathways developed in the main structurally active zones which favoured the emplacement of the granites. However, there is no evidence of genetic link between gold ores and their enclosing granites.

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

The magnetic properties of stage-2 Co c M 1− c Cl 2 graphite intercalation compounds (GICs) (M = Ni and Mn, 0 ≤ c ≤ 1) and their bulk intercalants, Co c M 1− c Cl 2, have been studied by using d.c. magnetic susceptibility. In stage-2 Co c M 1− c Cl 2 GIC, the adjacent Co c M 1− c Cl 2 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 c M 1− c Cl 2 GIC is very different from that of Co c M 1− c Cl 2. Stage-2 Co c Ni 1− c Cl 2 GIC with 0 ≤ c ≤ 1 and stage-2 Co c Mn 1− c Cl 2 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 c Mn 1− c Cl 2 GIC and Co c Mn 1− c Cl 2 are also compared with those of stage-2 Co c Mg 1− c Cl 2 GIC and Co c Mg 1− c Cl 2.

Surface structures of layered compounds treated with alkali-metal hydroxide solutions studied by scanning tunneling microscopy

Ordered 2 × 2 and √3 × √3 superstructures with charge density modulation were observed by scanning tunneling microscopy (STM) on highly oriented pyrolytic graphite surfaces treated with MOH (M=Na, K) aqueous solutions. Charge transfer from intercalants to host graphite was observed, as previously seen on stage-1 alkali-metal intercalations. On the other hand, for the MoS2 crystal treated with the same solutions, the host MoS2 lattice was observed together with island structures with low charge density formed by intercalants. Some electronic states seemed to be formed between the MoS2 band gap.

Effect of turbostratic disorder in graphitic carbon hosts on the intercalation of lithium.

The amount of lithium, ${\mathit{x}}_{\mathrm{max}}$, which can be intercalated in a graphitic carbon host is affected by the amount of turbostratic disorder in the host. Using electrochemical methods, we show that ${\mathit{x}}_{\mathrm{max}}$ in ${\mathrm{Li}}_{\mathit{x}\mathrm{max}}$${\mathrm{C}}_{6}$ is given by ${\mathit{x}}_{\mathrm{max}}$=1-P, where P is the probability of finding a random rotation or translation (turbostratic disorder) between adjacent graphite layers, implying that the interlayer spaces or galleries between randomly stacked layers do not host lithium ions. These ``blocked '' galleries influence the staged phases and the compositions of stage-1 (${\mathit{x}}_{\mathrm{max}}$) and stage-2 (${\mathit{x}}_{2\mathrm{m}\mathrm{a}\mathrm{x}}$) materials. A simple model is proposed to explain the variation of the composition of the stage-2 phase, ${\mathit{x}}_{2\mathrm{m}\mathrm{a}\mathrm{x}}$, with P.

Structural evolution and gold mineralization in the Scotchmans fault zone, Magdala gold mine, Stawell, western Victoria, Australia

The mid-greenschist facies turbiditic and subaqueous volcaniclastic metasediments at Stawell, Western Victoria, are overprinted by two shear zone systems that host gold mineralization in the Magdala mine. The younger Scotchmans fault zone overprints the older Central lode system and both systems have a reverse sense of movement. There is a systematic reorientation of the Central lode system structures and foliation into parallelism with structures of the Scotchmans fault zone. The boundaries of the Scotchmans fault zone are defined by discrete master faults along which are emplaced gold-bearing laminated quartz veins. The hanging-wall and footwall master faults are linked by an irregular array of subsidiary faults which form duplex structures containing slickenlines, rotated shear zone foliations, and S-C fabrics. Fault breccias, developed on both master and subsidiary faults, are overprinted by a fault gouge implying multiple movements along these faults. In zones where the subsidiary faults intersect and link with the master faults, gold grades increase from an average of 4 to 5 g/t in the laminated quartz veins to greater than or equal to 10 g/t.Microfabrics in the laminated quartz veins indicate vein accretion from either wall or from the center and quartz fibers attesting to vein growth occurring as a series of incremental events rather than being a single event. The gold-bearing fluids overprint earlier laminations but are coeval with later vein accretion associated with sulfides and ankerite. The mineral assemblage of pyrrhotite-pyrite-chalcopyrite-galena is deformed together with the host graphite mica schists and hosts no gold at all. This assemblage is overprinted by the shear zone-forming events. Subsidiary faults exhibit fault jogs in which quartz, pyrite, arsenopyrite, and minor chalcopyrite have precipitated. The assemblage of pyrite-arsenopyrite-chal-copyrite, which is always associated with the gangue minerals of quartz, ankerite, and minor calcite, is present in the laminated quartz veins and hosts the gold mineralization. The gold occurs typically as inclusions or in fractures within pyrite and arsenopyrite. Retrograde minerals are present, associated with the formation of pressure solution structures such as stylo-lites and quartz fibers overprinting the gold-bearing ore assemblages. Tetrahedrite and enargite replace pyrite and arsenopyrite, and some new pyrite has recrystallized from older grains. In the retrograde assemblages no gold is enclosed within individual minerals but it occurs on grain boundaries. This low-temperature mineral overprint is due to diffusive mass transfer that has remobilized the gold that was locked in pyrite and arsenopyrite as inclusions. Extensive dissolution of pyrite and arsenopyrite has resulted in gold redeposition into fractures, cracks, grain boundaries, and stylolites.

Chemical charging and electrochemical discharging through graphite intercalation compounds with sulfuric acid

The chemical charging and following electrochemical discharging through the formation and decomposition of graphite intercalation compounds with sulfuric acid were studied on three host materials with different textures, viz., natural graphite flakes, vapor-grown graphite fibers and mesophase-pitch-based graphite fibers. In the course of electrochemical discharging in sulfuric acid containing nitric acid as oxidant, a competition between the de-intercalation and re-intercalation of sulfuric acid into the graphite gallery was observed and, as a consequence, a strong effect of the texture of the host graphite on the discharging process. On the meshophase-pitch-based graphite fibers with radial arrangement of the graphite basal planes in the cross-section of the fibers, electrochemical discharging with rather steady state voltage in long period was possible.

CDW in FeCl4−—Graphite Intercalation Compounds Studied by STM

Abstract Samples of graphite electrochemically intercalated with FeCl4 − in CH3NO2 solution are studied by scanning tunneling microscopy (STM) from a submicrometer to atomic scale. A new long range triangular modulation of the charge density is observed on the surface. This modulation is superimposed on the usual graphite periodicity. This new structure is tentatively attributed to a charge density wave (CDW) resulting from the charge transfer between the intercalated species and the graphite host.

On The Relevance of Certain Transport-Structure Correlations IN SBCL5-Intercalated Graphite TO OUR Overall Understanding of GICc Axis Conductivity

This paper presents some new data on the SbCl5-intercalated graphite family as concerns the relationships between c axis resistivity, ρc(T,p), and intercalate layer structure. Results on the influence of intercalate layer crystallization and the nature of the host graphite are discussed and compared with those on other GIC families. Certain data are examined in the light of available theories and such an analysis raises a number of questions.

Graphite intercalation compounds and symmetry—I. Simple group

The symmetry groups of the graphitic host of a series of intercalation compounds are studied. The K vector groups, the compatibilities and the symmetrized functions are related in view of bond theory calculations.

Intercalation of AlCl 3 into FeCl 3-graphite intercalation compounds and occurrence of BI-intercalation

The formation process of the ternary AlCl 3-FeCl 3-graphite intercalation compounds (GICs) was studied by using three different reaction paths at 150°C. In the reaction of stage 1, FeCl 3-GICs with AlCl 3, the exchange of intercalated FeCl 3 for AlCl 3 occurred and, consequently, the formation of the “mixed”-type ternary GIC was observed at the edge of the flakes. The reaction of stage 2 FeCl 3-GIC with AlCl 3 however, gave the “bi-intercalation”-type GIC due to intercalation of Aid, into unoccupied graphite galleries. In the reaction of the host graphite with an equimolar mixture of AlCl 3 and FeCl 3 the preferential intercalation of AlCl 3, was observed in the beginning of the reaction, and then the exchange of AlCl 3, for FeCl 3 in the stage 1 compound occurred. Through the determination of the charge density distribution and the structural refinement by a pattern-fitting technique on mixedand bi-intercalation-type GICs, the former was found to have more defective intercalate layers than the latter.

Scanning tunneling microscopy study of ternary alkali-metal graphite intercalation compounds

Scanning tunneling microscopy (STM) has been applied to investigate the submicrometer and atomic structure of ternary alkali-metal graphite intercalation compounds (GIC's) in a high-purity inert gas environment. Here we focus on ternary stage-1 GIC's as MM'C 8 (M, M' =K, Cs, Rb). On a submicrometer scale, we observe islands of decreased or increased apparent topographic height, suggesting an inhomogeneous alkali-metal distribution on a lateral scale of a few tens of nanometers. Variation of the local work-function as a possible explanation is discussed. On the atomic scale, both hexagonal 2x2 superlattices and one-dimensional linear structures similar to stage-1 CsC 8 and RbC 8 are observed. In addition, a novel non-hexagonal superlattice rotated at an angle of 18° respective to the graphite host lattice with a periodicity of about 1.9 nm has been observed.

Transport properties in graphite intercalation compounds with transition metal fluorides

Disorder-induced phenomena are evidenced in both electrical and thermal transport properties of various graphite host materials with transition metal fluoride (C xMF y with M = Cr, Rh, Au and Ir). The electrical resistivity results are analyzed in the framework of weak-localization and carrier-carrier interaction models for two-dimensional disordered electron systems. Compared to other acceptor compounds, these fluoride GICs exhibit essentially two new features. First, for some highly resistive compounds, the logarithmic increase on resistivity with decreasing temperature starts already at room temperature and is dominant down to the lowest temperature investigated (1.5K). Second, the effect of disorder is also observable in the temperature dependence of the thermal conductivity and thermoelectric power.

New evidence for supported metals in the reaction of KC8 with metal chlorides

The evolution of the reaction between the binary graphitide KC8 and CoCl2 dissolved in anhydrous tetrahydrofuran (THF) has been investigated by in situ x-ray diffraction (XRD). During the reaction and whatever the host graphite (powder or HOPG), the first stage graphite intercalation compounds K(THF)2.5C24 and K(THF)1.7C24 are first formed and then transformed into higher stages. Finally the matrix is exfoliated and the phases identified are graphite, metallic cobalt, and KCl. Analyses by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) clearly show that the products are mainly located at the edge planes of the graphite, indicating that the electronic exchange occurs at the edge of the graphene layers. It is now clear that graphite-cobalt intercalation compounds cannot be obtained by this method.