A. Herold

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.

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.

Synthesis and electrical resistivity of lithium-pyrographite intercalation compounds (stages I, II and III)

Abstract Lithium has been intercalated for the first time into highly oriented pyrographite (HOPG) yielding first to third stage products. Their electrical resistivity has been studied as a function of stage and temperature from 100 K to 300 K. The results are discussed with respect to the graphite - heavy alkali metal compounds.

Thermal stability and structure of graphite-antimony pentachloride compounds

It is shown that antimony chloride reacting with graphite forms lamellar compounds C12n SbCl5 (n = 1,2,3,4,…). The identity period along the c axis is Ic = 9.42 A for the first stage and Ic = 9.36 + (n −1)3.36 A for the other stages. Electron diffraction and X-ray studies of the hk0 reflexions with a goniometric apparatus permit to specify the structure of the compounds of n > 2. The single graphite crystals are fragmented into many ordered fields which are themselves rotated by 60°. The structure of the carbon layers remains unchanged after intercalation. The intercalated antimony pentachloride forms a hexagonal system: a = 17.23 A (2.46 A × 7) the a axis being the same as the one for graphite.

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.

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.

Intercalation of rare earth metals in graphite

Abstract Like lithium and alkaline-earth metals, some lanthanides are able to give intercalation compounds with graphite. The intercalation is obtained either by action on the graphite of the vapor of the metal or by compression and heating of mixtures of graphite and metal powders. The structure of the 1st stage compounds EuC 6 and YbC 6 was determined with oriented samples by separate study of the different families of reflections: 00 l with samples prepared from prolytic graphite, hk 0 and hkl with single crystals. The hexagonal unit cell of these 1st stage phases belongs to the space group P6 3 /mmc with two metal atoms in position b and 12 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 ).

Etude structurale du graphiture I de cesium

Abstract The metallic yellow compound CsC8 belongs to the 1st stage. The formula CsC8 involves the occupation, in each interlayer, of only one in four possible sites the compact structure would entail the composition CsC2 (Fig. 1). From single crystal photographs, one can determine the c parameter as three times the identity period along the c axis: Ic. The structural determination is based on separate study of each family: 00l (Table 1, Fig. 2), hk0 and hkl (Table 2, Fig. 3) and powder diagrams (Table 3, Fig. 4). The hk0 family allows to determine the symmetry of the unit cell and the parameter; the systematic extinctions of the hkl reflections permit to determine the space group. The stacking of the cesium atoms: three in the four possible sites involves a screw axis 62 or 64. The position of the carbon and cesium atoms takes into account all the symmetries of the cells corresponding to the space group P6222 (or P6422) (Fig. 5). The foregone extinctions of those unit cells are respected, in particular, the 201, 202, 204…, reflexions are missed (Fig. 6). The two domains built on αβγ and αγβ are actually symmetric to a mirror and not superimposable. It is shown on Fig. 8 that there are two enantiomorphous structures: the stacking αβγ corresponds to an obverse screw axis for the cesium atoms. The presence of 6 different stacking zones even in a simple crystal involves a disorder in the compound which can explain the variations of intensity of the reflexions hk0 and hkl. The CsC8 compound is described by an hexagonal unit cell belonging to the space group P6222 (or P6422) with 24 carbon atoms in position 12k (x = 1 6 , y = 1 3 , z = 1 3 ) and 6i (x = 1 6 , z = 0 and x = 1 3 , z = 0) and the cesium atoms in 3b. The parameters are a = 4.945 ± 0.01 A and c = 17.76 ± 0.03 A . Two unit cells are mentioned, according to the direction of the screw axis. In fact, there are 6 different possible unit cells. We tried to imagine the different possibilities of stacking for the cesium atoms between the graphite layers. From an ideal graphite plane, one places the 1st cesium layer in position α. The second metallic layer can be placed on the β, γ or δ sites. Let us imagine a part in β position, an other in γ and the last one on δ. In the same graphite matrix, one can define three distinct domains spotted by the successions αβ, αγ and αδ. While the filling of the last cesium layer, each domain is divided in two and it appears 6 zones, which correspond to the stacking αβγ, αβδ; αγβ, αγδ; αδβ, αδγ (Fig. 7). Each of these domains is defined by an hexagonal unit cell. They differ one from each other.