D. Guerard

Intercalation of lithium into graphite and other carbons

Abstract Lamellar graphite-lithium compounds of the first, second, third and fourth stages which are respectively brass-yellow, steel-blue, dark-blue and black had been prepared by the two following methods: 1. (a) by heating graphite and lithium at 400°C in a copper or stainless-steel tube; 2. (b) by compressing lithium powder with crushed natural graphite under an argon atmosphere in a glove-box. The reaction begins at room temperature. An homogeneous product is obtained by annealing the pellet at 200°C in vacuum or in argon. Whereas the first method leads only to the first stage compound LiC6 which is stoichiometric, pure phases of any stage may be obtained according to the composition of the initial mixture by the second method. The analysis of pure phases shows that the compounds of stages n > 1 present large variations of the stoichiometry. The unit cells of first stage LiC6 and second stage LiC12 compounds belong to P6/mmm space group with respective parameters a = 4.305 ± 0.001 A, c = 3.706 ± 0.01 A and a = 4.288 ± 0.002 A, c = 7.065 ± 0.02 A. Lithium intercalates also into soft and hard carbons, including fibers, like the heavy alkali metals.

One-step synthesis of maghemite nanometric powders by ball-milling

Iron powder was milled within water for different duration using a planetary ball mill equipped with stainless steel vials. The in-situ production of hydrogen hinders the hematite formation during the grinding. X-ray diffraction, chemical analysis, high resolution transmission electron microscopy (HRTEM) and Mossbauer spectroscopy reveal that the obtained nanostructured powders consist of maghemite. Direct synthesis of maghemite nanoparticles from iron powder is so realised. Particles of about 15 nanometers are obtained after 48 h of milling.

New results about the sodium-graphite system

Abstract For a long time, it was accepted that the intercalation of sodium in graphite was possible only in the presence of impurities; the best compound obtained at high temperature was described by Asher; it belongs to the eighth stage with a formula NaC64. Based on new considerations, we tried to prepare lower stages. It is possible to obtain small amounts of stage four mixed with unreacted graphite. Pure sixth and seventh stages were prepared. The 00l reflections analysis allow the determination of the interplanar distance between two carbon layers surrounding a sodium plane: 4.52 A. A study of the influence of the temperature of preparation shows that the lowest stages are obtained at very low temperature, apparently in the absence of impurities.

L'insertion dans le graphite des amalgames de potassium et de rubidium

Abstract Potassium and rubidium amalgams are able to intercalate into graphite and produce ternary compounds of two types. With amalgams rich in alkali metal, binary compounds with small amounts of mercury in the intercalated layers are obtained: MC 8 (Hg). With amalgams of composition around MHg, one can prepare ternary multilayered compounds named mercurographitides: in these compounds, the intercalated layers consist of two alkali metal planes with a sheet of mercury in between. The data for the first stage mercurographitides MHgC 4 are given in Table 2. The identity period along the c axis was obtained from X-rays data (Fig. 1, Table 1). The hk 0 reflexions show the octal epitaxy of the metal layers between the carbon planes (Fig. 2, Table 3). The intercalation of the amalgam occurs in two steps: first a quasiselective intercalation of the alkali metal (Fig. 3) and then a simultaneous intercalation of mercury and alkali metal (Fig. 4). The first step gives the compound MC 8 (Hg) and the second MHgC 4 . The second stage mercurographitides MHgC 8 are characterized by their identity period along the c axis (Fig. 5, Table 4) and are given in Table 5. A mechanism of addition of mercury to the MC 8 phases is proposed in Fig. 6. Finally, in an effort to prepare mercurographitides of stages higher than 2, the identity period along the c axis for the third stage compound was found, although this compound was not obtained in a pure state.

Ball-milling: the behavior of graphite as a function of the dispersal media

Abstract The ball-milling in liquid media leads to well organized, thin and highly anisometric graphite (HAG) crystals. The presence in the milling container of a liquid, which acts as a lubricant and decreases the violence of the shocks, is relevant. Two liquids are used: n-dodecane and water. With dodecane, inert towards graphite and the metal of the milling tools, the powder consists of pure graphite whereas with water, the graphite particles are covered with nanocrystallites (15 nm) of a magnetic compound: the maghemite (γ Fe2O3). The electrochemical properties of those powders are interesting. The highly anisometric graphite leads to an irreversible capacity around half of that for the initial graphite powder, in contradiction with previous results claiming that higher the surface area, the higher the irreversible capacity. In fact, milling in the presence of dodecane provokes essentially a cleavage, which increases the global area, but does not drastically change the number of edge carbon atoms, responsible for the increase of the large irreversible capacity. The graphite–maghemite composites present a high capacity, partly reversible by oxidation–reduction between iron and wustite (FeO). This reaction is made possible by the nanometric size of the particles, and therefore their high reactivity.

Insertion de metaux alcalino-terreux dans le graphite

Resume The pure first stage strontium and barium graphitides were prepared using two different methods: 1. (1) direct action of the metal vapor on the graphite, in metallic sealed tubes, 2. (2) compressing of the powders and heating of the mixture. From the physical properties of the alkaline Earth metals, it appears that the first method is favourable in most cases (Table 1). The formula MC 6 was clearly established from the chemical analysis (Table 3), weight uptake (Table 4) density (Table 5) and also from the planar unit cell (Fig. 2). The structure was obtained by a technique which allows to study separately the different families of reflexions: —001 from pyrolytic graphite (Table 6, Fig. 4); — hk 0 and Khl from single crystals. The relative intensities of the hk 0 and hkl reflexions (Table 7–9), and the systematic extinctions (Fig. 5) are in good agreement with the space group P 6 3 / mmc with two metal atoms in position b and twelve carbon atoms in i (Table 10, Fig. 3). The indexation of all reflexions in a powder X-ray diagram confirms this structure.

X-ray investigation of highly saturated Li-graphite intercalation compound

Abstract Highly saturated lithium-graphite intercalation compounds (of a composition LiC2LiC4) synthesized under high-pressure conditions were investigated using X-ray diffraction. It was shown that these compounds present the structure with hexagonal unit cell with a parameter 8.63 A , c = 3I c = 3 · 3.7 = 11.1 A . The most probable stoichiometry for this unit cell is LiC2.67, as deduced from the comparison of the calculated and observed intensities of (hkl) reflections. It is supposed that this structure is the most “stable” step in decomposition of LiC2 compound obtained under pressure.

Insertion de lanthanoides dans le graphite

Abstract The intercalation of lanthanides is possible by two different ways: direct action of the metal vapor in metallic tubes sealed under vacuum or by heating compressed mixture of powders of the metal and of graphite. The first method was used only for the most volatile metals: samarium, europium, thulium and ytterbium (see Table 1). The 00 l reflexions of the corresponding first stage graphitides MC6 (M = Sm, Eu, Tm or Yb) were obtained for samples prepared from pyrolytic graphite HOPG (Tables 2 and 3, Fig. 1). Using the second method, we determined the identity period along the c axis (Ic) for the most fusible lanthanides graphitides (Table 4). We did a study of the evolution vs temperature of a compressed mixture of initial composition Yb + 6C (Fig. 2). A structural study was carried out on single crystals of europium and ytterbium graphitides, using a special method developed in the laboratory. Figure 3 shows the three possible types of sites available to the metal atoms and the different stacking modes of the metallic layers. The hkO and hkl reflexions of the graphitide YbC6 (see Table 5) shown in Fig. 5 allow to choose between the three possible unit cells of Fig. 4. The hexagonal unit cells of EuC6 and YbC6 belong to the space group P63/mmc with two of three sites occupied regularly by the metal atoms. The two metal atoms are in position b and the twelve carbon atoms in i. The parameters are respectively a = 4.314 ± 0.003 A , c = 9.745 ± 0.008 A (EuC 6 ) and a = 4.320 ± 0.004 A , c = 9.147 ± 0.004 A (YbC 6 ) .

Intercalation of the amalgams KHg and RbHg into graphite: Reaction mechanisms and thermal stability

Abstract The intercalation of the amalgams MHg (M = K, Rb) allows the synthesis of the first stage MHgC4 and the second stage MHgC8 ternary compounds. The intercalant is formed by two layers of alkali metal surrounding an intermediate plane of mercury atoms. We have shown that the intercalation occurs in two steps. (i) A quasi-selective intercalation with a continuous change of stage where only the alkali metal penetrates between the graphite planes. This step leads to the MC8 compound with some mercury atoms in the metallic sheets. (ii) A simultaneous cooperative intercalation: alkali metal and mercury penetrate together in the occupied interlayers with the ratio Hg:M = 2:1 to build the three layered intercalant MHgM. The formation of the second stage compounds from the MC8 binaries involves an addition of mercury atoms with a reorganization of the metallic layers in the graphite interval. On the other hand, the intercalation isobar and isotherm curves show that the MHgC4 compounds have a very small stability domain when they are in presence of an excess of free amalgam in a temperature gradient. When this gradient exceeds a few degrees, these ternary compounds decompose in the binary MC8 compounds.

Superconductivity of the graphite intercalation compounds KHgC8 and RbHgC8: Evidence from specific heat☆

Abstract Low temperature specific heat measurements on the second stage graphite intercalation compounds KHgC8 and RbHgC8 show anomalies characteristic of superconductivity at 1.93 K and 1.46 K respectively. The electronic density of states, the Debye temperature and the electron-phonon coupling constant are obtained and comparison with a.c. susceptibility studies is made.